The region of Diwalwal, dominated by Mt. Diwata, is a gold rush area on Mindanao (Philippines) where approximately 15000 people live. The fertile plain of Monkayo is situated downstream, where people grow crops such as rice and bananas; locally caught fish is eaten frequently. The ore is dug in small-scale mines and ground to a powder by ball-mills while still in Diwalwal. The gold is then extracted by adding liquid mercury (Hg), forming gold-amalgam. To separate the gold from the Hg, in most cases the amalgam is simply heated in the open by blow-torches. A high external Hg burden of the local population must be assumed. To evaluate the internal Hg burden of the population and the extent of possible negative health effects, 323 volunteers from Mt. Diwalwal, Monkayo and a control group from Davao were examined by a questionnaire, neurological examination and neuro-psychological testing. Blood, urine and hair samples were taken from each participant and analyzed for total Hg. A statistical evaluation was possible for 102 workers (occupationally Hg burdened ball-millers and amalgam-smelters), 63 other inhabitants from Mt. Diwata ('only' exposed from the environment), 100 persons, living downstream in Monkayo, and 42 inhabitants of Davao (serving as controls). The large volume of data was reduced to yes/no decisions. Alcohol as a possible bias factor was excluded (level of alcohol consumption and type, see Section 4.4). Each factor with a statistically significant difference of at least one exposed group to the control group was included in a medical score (0-21 points). In each of the exposed groups this score was significantly worse than in the control group (median control, 3; downstream, 9; Mt. Diwata, non-occupational exposed, 6; Hg workers, 10). In comparison to the surprisingly high Hg concentration in blood (median, 9.0 microg/l; max, 31.3) and in hair (2.65 microg/g; max, 34.7) of the control group, only the workers show elevated levels: Hg-blood median 11.4, max 107.6; Hg-hair median 3.62, max 37.8. The Hg urine concentrations of the occupational exposed and non-exposed population on Mt. Diwata was significantly higher than in the control group: control median 1.7 microg/l, max 7.6; non-occupational burdened median 4.1, max 76.4; and workers median 11.0, max 294.2. The participants, living downstream on the plain of Monkayo show no statistically significant difference in Hg-blood, Hg-urine or Hg-hair in comparison with the control group. The German Human-Biological-Monitoring value II (HBM II) was exceeded in 19.5% (control), 26.0% (downstream), 19.4% (Mt. Diwata, non-occupational) and 55.4% (workers) of the cases, the German occupational threshold limit in 19.6% of the workers. Only some of the clinical data, characteristic for Hg intoxication (e.g. tremor, loss of memory, bluish discoloration of the gingiva, etc.), correlate with Hg in blood or urine, but not with Hg in hair. The medical score sum correlates only with Hg in urine. The poor correlation between the Hg concentration in the biomonitors to classic clinical signs of chronic Hg intoxication may be explained by several factors: Hg in blood, urine and hair do not adequately monitor the Hg burden of the target tissues, especially the brain. Inter-individual differences in the sensitiveness to Hg are extremely large. In this area a mixed burden of Hg species must be assumed (Hg vapor, inorganic Hg, methyl-Hg). Chronic Hg burden may have established damage months or even years before the actual determination of the Hg concentrations in the bio-monitors under quite different burden was performed (Drasch G. Mercury. In: Seiler HG, Sigel A, Sigel H, editors. Handbook on metals in clinical and analytical chemistry. New York: Marcel Dekker, 1994:479-494). Therefore, a 'Hg intoxication', that should be treated, was not diagnosed by the Hg concentration in the bio-monitors alone, but by a balanced combination of these Hg values and the medical score sum. In principle, this means the higher the Hg concentration in the bio-monitors, the lower the number of characteristic adverse effects are required for a positive diagnosis. By this method, 0% of the controls, 38% downstream, 27% from Mt. Diwata, non-occupational exposed and 71.6% of the workers were classified as Hg intoxicated. A reduction of the external Hg burden on Mt. Diwata is urgently recommended. An attempt to treat the intoxicated participants with the chelating agent dimercaptopropanesulfonic acid (DMPS) is planned.