To study the prevalence of carpal tunnel syndrome (CTS) in the general population and the value of brachialgia paraesthetica nocturna (BPN) in diagnosing CTS, an age and sex stratified random sample of 715 subjects was taken from the population register of Maastricht (The Netherlands) and surrounding villages, between September 1983 and July 1985. The response rate was 70%. Of these, 12 CTS cases had already been diagnosed. Of the remaining subjects, 64 (13 men, 51 women) woke up because of BPN. Among these subjects 1 man and 23 women were found to have CTS. The prevalence rate of undetected CTS was 5.8% [95% confidence interval (CI): 3.5-8.1%] in adult women; 3.4 percent (95% CI: 1.5-5.3%) had already been diagnosed as CTS. The overall prevalence rate for men was 0.6% (95% CI 0.02-3.4%). These figures have to be regarded as minimal estimates. The overall diagnostic value of BPN for CTS was 38%, while for women only this was 45% (95% CI: 31-60%).