Copper can induce acute and chronic intoxications in humans. Copper in tap water has caused a series of severe systemic diseases in Germany in recent years (chronic copper poisoning, CCuP). From the clinical point of view it has been difficult to establish the diagnosis on the basis of clinical and laboratory methods. In a retrospective study, we therefore looked for essential clinical signs as well as laboratory findings which might be typical and essential for the diagnosis of CCuP. - We observed that in patients with severe systemic CCuP not only the liver but also several other organs have been the target of copper. As a proof copper overload has been measured. The latter results are presented here. - During or shortly after exposure "free" serum copper (= non-ceruloplasmin-bound copper) was significantly elevated in all patients (range 5.1 to 47.1 micromol/l, or 25.7 to 56.2 % of total serum copper). The normal upper limits in infants according to Salmenperä (8) are: 0.3 micromol/l, or 1.6 % of total serum copper. - Total serum copper was elevated in 14/16 patients: 13.7 to 30.1 micromol/l in sick infants (normal upper level: 12.6 micromol/l), and 17.0 to 27.2 in sick children (normal upper level for children and adults: 21.4 micromol/l). - Urine copper excretion was found elevated in 9/10 patients, with a range of 11 to 456 microg/dl (normal upper level in adults: 15 microg/dl). - Our results show that patients with systemic CCuP are in a "hypercupric" state. The data thus firstly prove that indeed the putative agent copper is found in excess in the patients and secondly show that the estimation of "free" copper in serum and the measurement of copper in urine are reliable diagnostic methods. Elevation of total serum copper (even though not specific) can give a first hint to the diagnosis. - The hypercupric state of systemic CCuP can be differentiated from that of Wilson's disease by (1) normal levels of ceruloplasmin and (2) the observation that values for free copper in serum or urinary copper normalize in an environment without copper in tap water, for instance in a hospital.