Previous studies of the effect of mother's employment on infant birth weight have shown mixed results. This study was designed to explore the relationship between activity level of a woman's occupation and her risk of delivering a low-birth-weight infant. Using 1981 state of Washington birth certificate records, the authors selected 5,822 subjects. Case subjects were women who had delivered infants weighing 2,500 g or less; controls were those who had delivered normal-weight infants. Women were grouped into five activity levels based on their usual occupation. No association was detected between activity level of mother's usual occupation and delivery of a low-birth-weight infant. These data suggest that active work is not associated with an increased risk of having a low-birth-weight baby.