This report describes psychomotor retardation in infants caused by prenatal exposure to methylmercury. A study of 29 mother-infant pairs established a relationship between maximum maternal hair mercury concentration during pregnancy and the frequency of neurological effects in the infants. The latter included delayed achievement of developmental milestones with or without neurological signs. The infants were 4 1/2 to 5 years of age on last examination. Ten infants of mothers who had maximum hair concentrations in the range of 99 to 384 pars per million (ppm) had a significantly higher frequency of abnormal findings than those in two groups having lower maternal hair mercury concentrations (12 to 85 and 0 to 11 ppm). The group sizes were too small to allow identification of a specific "threshold" maternal hair concentration above which such effects can be expected. Transient paresthesias during pregnancy occurred in 80% of the mothers in the higher concentration group (99 to 384 ppm) as compared with 30% and 22% in the lower groups. Neurological abnormalities were found in some children whose mothers had been asymptomatic during pregnancy.