This is a prospective study of prenatal substance use. Women were interviewed during their fourth and seventh months of pregnancy, at delivery, and at 8, 18, and 36 months postpartum. At birth, there were 763 liveborn, singleton offspring in the sample. At each phase, the offspring were examined and measured for growth. Data are presented on the relationship between tobacco and marijuana use and the size of the offspring at birth, 8, 18, and 36 months of age. At birth, there was a significant inverse relationship between tobacco use and weight, length, and head circumference. At 8 months of age, only length continued to be associated with prenatal tobacco exposure. By 18 months of age, there was no relationship between prenatal tobacco exposure and size of the offspring. Prenatal marijuana exposure was only associated with decreased length at birth. Neither tobacco nor marijuana use predicted gestational age or morphological abnormalities.