Congenital nephrosis of the Finnish type (CNF) is inherited as an autosomal recessive trait which maps to the long arm of chromosome 19. The disease causes massive proteinuria, and renal transplantation in early neonatal life is the only effective treatment. Prenatal diagnosis, usually in high-risk families, depends on the analysis of alphafetoprotein (AFP) levels in maternal serum (MS) or amniotic fluid (AF). We studied the effectiveness of MSAFP measurement as a method of screening all pregnant women for congenital nephrosis. Between 1 January 1979 and 31 December 1992, all pregnant women ( N = 110 858) attending maternity care units in East Finland were offered serum AFP measurement as part of a screening programme. All patients whose MSAFP value was 22.5 multiples of the median (MOM) at 15-18 weeks' gestation were given an ultrasound examination and if no morphological abnormality was found, they were then offered amniocentesis. Altogether, 105 880 pregnant women (96 per cent) in East Finland participated in the screening for CNF. A total of 47 cases of CNF were diagnosed during the study period. Elevated AFP concentrations in maternal serum and amniotic fluid (22.5 MOM) were found in all screened (441105 880) affected pregnancies except one. The most typical feature of a CNF pregnancy was a very high AFP concentration in the amniotic fluid. As a result of the screening, the prevalence at birth of CNF decreased from 1:2600 to 1:ll 086. The possibility of CNF has to be taken into account in pregnancies with a 'false-positive' elevated AFP result (normal ultrasound and no detectable acetyl cholinesterase).