In a prospective study the relationship of marijuana use during pregnancy to infant birth weight and length of gestation was examined in 583 women who delivered single live infants. Eighty-four women used marijuana to varying degrees during pregnancy. Marijuana, alcohol, smoking, and nutritional habits were estimated by repeated interviews. Compared to nonuse, an average use of marijuana six or more times per week during pregnancy was associated with a statistically significant reduction of 0.8 weeks in the length of gestation after consideration of the effects of nicotine, alcohol, parity, mother's prepregnancy weight, and the sex of the infant. With similar adjustments no reduction in birth weight was noted. Among the heavy marijuana users the effect of gestation length was does dependent.