A study was undertaken to determine the effects of tobacco smoke in the home on children's cognitive development. The study focused on 280 children, representing equal numbers of boys and girls and of Whites, Blacks, and Hispanics. When the participating children were 2 years old, their mothers were surveyed, interviewed, and tested to gather information on socioeconomic status, smoking habits, day care, infant feeding practices, intelligence, and the home environment. At age 3, the children completed the Stanford-Binet Fourth Edition (SBIV) measure of intelligence quotient (IQ) and the Peabody Picture Vocabulary Test. Data analysis revealed that the total number of packs of cigarettes smoked in the home was inversely related to child intelligence. Specifically, data indicated that as cigarette smoking increased by 1 pack per day, child IQ decreased by 1.65 points. Further analysis found that only the amount smoked by the mother was related to the children's IQ scores. Finally, data were also analyzed for 108 of the original children retested at 5 years of age, revealing that the particular relation of mothers' smoking habits to child IQ at 5 was not statistically significant. Study findings support the growing body of evidence that inhaling tobacco smoke from the environment has harmful effects on development. (AC)