For several years the Clinical Genetics Research Laboratory at Martin, Czechoslovakia, has been studying alkaptonuria (AU) in the northern part of the District of Trencín in Slovakia. These affected individuals are part of a group of 103 alkaptonurics originated mostly in the mountainous parts of Slovakia. We report results of pedigree analyses; population and affected-family biochemical urine screening; estimation of inbreeding coefficient, of exogamy rate and of average marital distance and of calculation of the frequency of the AU allele, and of homozygotes and heterozygotes in this portion of the Trencín District. Twelve homozygotes were found, but seven originated from a single hamlet in which a founder effect - genetic drift and inbreeding - are thought to account for the high prevalence of AU.