A sample of 2209 white South African gold miners aged 45-54 between 1968-71, who started mining exposure during 1936-43, was investigated from 1968-71 to 30 December 1986. The effect of silica dust and tobacco smoking on mortality from chronic obstructive lung disease (COLD) was assessed. The relative risk (RR) for dust exposure before 1950 was estimated as 2.5 (95% confidence interval (CI) 1.5-4.2), for 10 units of 1000 particle-years. The combined effect of dust exposure before 1950 and years of cigarette smoking on mortality from COLD was best estimated by the multiplicative model, indicating that the two exposures act synergistically. All those that died of the disease were smokers. According to the estimates of attributable risk about 5% of the deaths from COLD were from the effect of dust, 34% were from smoking, and 59% were from the combined effect of dust and smoking. In conclusion, the results indicate that workers exposed to silica dust who smoke are at higher risk of dying from COLD than smokers not exposed to silica dust, as the two exposures act synergistically in causing COLD.