Hyperthermia as a human teratogen has been implicated as one cause for neurulation defects. To determine whether there is an association between early maternal hyperthermia (20 to 28 days' gestation) and isolated occipital encephalocele, record reviews were conducted for the period 1969 through 1979 in three major medical centers in the Pacific Northwest. Control patients consisted of children with Down's syndrome matched for year of birth, sex, and race. Of the 17 patients ascertained with an isolated posterior encephalocele, four (24%) of the mothers gave a history of hyperthermia, due to prolonged fever of at least 1.5 C above normal thermal levels early in gestation. In the control patients and siblings of affected children, no history of maternal hyperthermia was elicited. These data are compatible with the concept that early maternal hyperthermia is one cause in the genesis of isolated occipital encephalocele.