To test a recent hypothesis on the causal relationship between sauna-induced hyperthermia and congenital defects, 100 consecutive cases of defects of the central nervous system and 202 cases of orofacial clefts were singled out from the Finnish Registry of Malformations. The mothers and their time-area-matched referents were interviewed for their sauna habits. Almost every pregnant mother (98.5%) had visited the sauna regularly, and yet, the incidence of the CNS defects in Finland is among the lowest ever reported. No differences in the sauna habits were observed between the case and referent mothers. It is concluded that the relatively mild, temporal hyperthermia caused by the sauna should not be considered hazardous for the developing embryo.