In January 1985 a smog period occurred for 5 days in parts of West Germany, including the Rhur District. Mortality (24,000 death certificates), morbidity in hospitals (13,000 hospital admissions, 5400 outpatients, 1500 ambulance transports) and consultations in doctors' offices (1,250,000 contacts) were studied for a 6-week period including the smog episode and a time interval before and thereafter. The study region was the State of North Rhine-Westfalia (16 million inhabitants), but the analysis is restricted to the comparison of the polluted area and a control area (6 million inhabitants each). During the smog period, mortality and morbidity in hospitals increased in the polluted area, but there was no substantial increase in the control area. The increases were for the total number of deaths 8 vs. 2% (polluted area vs. control area), for hospital admissions 15 vs. 3%, for outpatients 12 vs. 5% and for deliveries by ambulance to hospitals 28% in the polluted area (not investigated in the control area). The effects were more pronounced for cardiovascular diseases than for respiratory diseases. The consultations in doctors' offices show a slight decrease (-2 vs. -4%). Regression analysis shows a moderate influence of temperature, but a strong influence of ambient air pollution. The maxima of the ambient concentrations are more important on the same day, whereas the influence of the daily averages is more pronounced after a delay of 2 days. The results are discussed considering other possible confounders such as indoor pollution and psychogenic influences of the alarm situation. In total, the study suggests moderate health effects due to increased air pollution during the smog episode.