Authors report on the genetic epidemiologic investigation of one of the sentinel anomalies made on purpose to define the birth prevalence of the different types of it, to calculate the mutation rate of autosomal dominant forms as well as to recognize the clinical features of Hungarian cases. 58% of all cases registered in the Hungarian Congenital Abnormality Registry in 1975-1984 proved to be atypical. The birth prevalence of autosomal dominant forms was 1.33/100.000 total birth. 71% of dominant forms was sporadic since 29% was familial. Thus the mutation rate was estimated 4.7 x 10(-6) +/- 1.22 x 10(-6).