We conducted a time-series analysis of daily hospital visits and air pollution data to assess acute effects of air pollution on daily unscheduled outpatient visits to internal medicine, pediatric, and emergency departments in the No. 3 Affiliated Hospital of Beijing Medical University in Beijing, China. Sulfur dioxide was marginally significantly associated with total outpatient visits (beta = 41.5, SE = 24.2) and significantly associated with internal medicine (beta = 14.6, SE = 6.7), pediatric (beta = 12.7, SE = 3.7), and emergency room visits (beta = 6.8, SE = 2.7). Total suspended particulates (TSP) was a significant predictor for total outpatient (beta = 21.1, SE = 7.7) and pediatric visits (beta = 3.4, SE = 1.3) and a marginally significant predictor of internal medicine visits (beta = 4.2, SE = 2.2). In a season-specific analysis, SO2 was a significant predictor for total hospital outpatient visits in summer, although the mean daily SO2 concentration was only 17 micrograms/m3 (maximum = 51 micrograms/m3). In winter, SO2 was significantly associated with internal medicine, pediatric, and emergency room visits, and TSP was associated with total outpatient visits. This study suggests an exposure-response relationship between TSP and SO2 and hospital outpatient visits, both at high air pollution levels and at levels well below air quality standards recommended by the World Health Organization.