Sentinel phenotypes are indicators of germinal dominant gene mutations. 23 sentinel abnormalities and 2 sentinel childhood tumours (bilateral retinoblastoma and Wilms' tumour), i.e., 25 sentinel anomalies of autosomal dominant origin were selected from the material of the Hungarian Congenital Malformation Registry, 1980-1984. Furthermore, cases of sentinel childhood tumours from the Hungarian Childhood Tumour Registry and some extra notifications of sentinel abnormalities were also included. Experts examined index patients and their parents in order to confirm or exclude nosological diagnosis, to separate sporadic and familial cases, to obtain environmental history and to give genetic counselling. The revised total observed prevalence of 25 sentinel anomalies was 3.80 per 10,000 livebirths. Only 12% of cases examined were familial. According to the statistical power calculation, 47,500 livebirths are needed to detect a doubling of mutation rate with probabilities of type I and II errors of 0.05 level. In Hungary the average number of yearly births was 135,548 in the study period.