In a sample of 3- and 5-yr.-old children, smoking in the home was found to be significantly and inversely related to IQ. Children of normal birth weight and without neurological impairment had been enrolled in a longitudinal study of child development. Analyses were conducted with sex, ethnicity, socioeconomic status, educational stimulation in the home, day care, and mother's intelligence controlled. Significant results were obtained for scores on the Peabody Picture Vocabulary Test-Revised at age three years and on the major Stanford-Binet Fourth Edition scales at ages three and five years. All effects were for the mother, not the father, smoking in the home.