As part of a general health screening survey in the Burgh of Renfrew blood pressure was measured in 3,001 subjects (78.8% of those eligible) aged 45 to 64. In 468 (15.6%) diastolic blood pressure was 100 mm Hg or more. A year later the mean blood pressure for those of the population re-examined showed no change, there being an equal number of subjects with increased and decreased pressures. The prognostic significance of those showing the larger fluctuations remains to be determined through medical-record linkage.Examination of the general practitioners' medical records of 422 of the 468 subjects with diastolic blood pressure of 100 mm Hg or more showed that 255 had no previous documented hypertension. Of the remainder 73 were receiving antihypertensive therapy. Examination of the records of subjects whose blood pressure was under 100 mm Hg showed that 55 were receiving antihypertensive treatment and that another 113 had previously been recorded as having a diastolic blood pressure of 100 mm Hg or more by their general practitioner. Altogether at least 636 (21.2%) of those who were examined had been considered at some time to have evidence of hypertension.The prevalence of undetected hypertension in the general population has important implications for the resources of the National Health Service if current trials show benefit to the health of the community from treating "mild" as well as "moderate" hypertension.