The Illinois Department of Public Health, in cooperation with the Centers for Disease Control (CDC), monitors trends in the prevalence of prenatal risk factors that are major predictors of infant mortality and low birth weight (LBW). Analyzed data from CDC are available to the department annually. During 1988, a total of 26,767 records of Illinois women giving birth were submitted to CDC. These surveillance data support the fact that women older than 30 years who smoke and enter pregnancy underweight are at greatest risk of delivering LBW babies. Overall, 13.9 percent of underweight smokers had LBW infants compared with 8 percent of underweight nonsmokers. Prevalence of LBW among underweight and smoking women older than 34 years was much higher (29.6 percent) than among those between ages 30 and 34 (15.2 percent). The prevalence of LBW decreased as the pregravid weight increased among normal weight smokers (10 percent) and overweight smokers (8.6 percent).