AbstractThis study examined the combined effects of maternal smoking and maternal age on birth weight. A sample of 1,851 white, full‐term infants (37 to 42 weeks) born to primiparous mothers ranging in age from 18 to 41 years was derived from the database of the Child Health and Development Studies (CHDS), available through the National Technical Information Service (NTIS) of the U.S. Department of Commerce. The mothers were classified into three age groups—young, middle‐aged, and older‐aged—corresponding respectively to values below the 15th, between the 15th and 85th, and at or above the 85th percentile of age.Analyses of the data show that the reduction in birth weight associated with maternal smoking when compared with nonsmoking averaged 181 gm for the young group (18 to 20 years), 162 gm for the middle‐aged group (21 to 29 years), and 154 gm for the older‐aged group (30 to 41 years). These differences are independent of total income. Hence it appears that the birth weight–reducing effect of smoking is not enhanced among women of older reproductive ages (30 to 41 years). Furthermore, the reduction in birth weight is dose responsive. That is, the greater the number of cigarettes smoked per day, the lower is the resulting mean birth weight and the higher is the proportion of low‐bir