OBJECTIVE The objective of this study was to estimate the prevalence of Huntington's disease in order to devise a programme for diagnosis and prevention. Because of the characteristics of this disease, which is hereditary and of low incidence, few epidemiological studies have been carried out in Spain. Many studies (Medline 1990-1996) give cross-checking of registers as the key to determining the relative extent of the illness. The findings of this comparison of registers are not limited to the numerical quantification of a health problem, but are combined with active case search strategies, since there is now an approximation of probability to a previously unknown area of the disease. METHODS Prevalence in the Valencia Region (Spain), which has a population census of 3,873,812 inhabitants, was estimated by means of the probability method known as capture-recapture. The estimated maximum probability and its confidence interval were calculated. The sources of information used were clinical histories from the regional hospitals and official figures of registered deaths during the period 1987-1992. RESULTS It was found that there were 41 cases seen in the regional hospitals and 17 deaths recorded in the official statistics, while 4 cases coincided in both sets of statistics. Recovery of 45 cases histories enabled an analysis to be made of the relationship between the disease, sex, age of onset of symptoms, and family history. CONCLUSIONS The estimated prevalence was 5.38 x 10(5). The most notable finding was that of a systematically earlier onset of symptoms in women, which was greater when the family history was on the paternal side.