Recent studies have suggested that exposure to air pollution might be associated with low birth weight. The effects of sulfur dioxide (SO(2)) and particulate matter less than 10 microm (PM(10)) were examined on birth weight in each trimester of pregnancy. The study group included all full-term singleton live births during 1995-1997 to women living within about 2 km of an air pollution monitoring site in Kaohsiung. Measurements of SO(2) and PM(10) collected at six air quality monitoring stations were used to estimate the influence of exposures on different pregnancy trimesters. This was done by averaging daily ambient air pollution concentrations during the corresponding days based on the birth date and gestational age of each child. Multiple linear regression analysis was used to estimate the effects of air pollution on birth weight adjusting for possible confounders including maternal age, season, marital status, maternal education, and infant gender. The estimated reduction in birth weight was 0.52 g for 1 microg/m (3) increase in either SO(2) or PM(10) in the first trimester of pregnancy. Data provide further support for the hypothesis that air pollution can affect the outcome of pregnancy.