A nationwide study on the Meckel syndrome (MS) was carried out covering retrospectively the years 1970-1979 and prospectively the years 1980-1981. Sixty-seven cases from 48 Finnish families were found. The clinical and pathological findings were studied. Cystic dysplasia of the kidneys was present in all the cases where sufficient information was available. In all 41 cases with specimens available from the liver, a typical fibrotic change with proliferation and dilatation of the bile ducts was found. This series and a review of the literature give convincing evidence that cystic dysplasia of the kidneys with fibrosis of the liver is a constant finding in the "true" Meckel syndrome. In conclusion, it is proposed that cystic dysplasia of the kidneys with fibrotic changes of the liver and occipital encephalocele or some other central nervous system malformation are considered as minimal diagnostic criteria of MS. Thus, a histologic investigation of the kidneys and liver is essential in diagnosing MS in doubtful cases. Heterozygote manifestations were not found.