OBJECTIVE The study evaluated the possibility of a direct association between silica dust exposure and lung cancer. METHODS Mortality and morbidity among 1026 granite workers was followed in 1940-1989. Regional census data for 1970-1985 and lung cancer incidence data were also linked. The cytotoxicity of different granite fractions and their capacity to induce reactive oxygen species (ROS) in human leukocytes was studied in vitro. RESULTS Excess lung cancer mortality was found during several follow-up periods. The rate ratios were 1.6-3.8 for different latency periods. Lung cancer risk increased with the length of exposure and latency. Lung cancer morbidity in 1953-1987 showed an excess for the red and grey granite areas, the rate ratio being 2.5 for > or = 20-year latency. Only one lung cancer case came from the black granite area. The cancer cases from the grey area had a shorter latency than those from the red area. The mineral composition differed for each area. The strongest ROS-inducing activity of grey and red granite was seen in the quartz-containing fractions. In the cytotoxicity tests the quartz-containing fractions of the grey and red granite also caused the strongest lactic dehydrogenase (LDH) release. However, plagioclase, the main constituent of black granite (60%), had approximately the same ability to induce ROS as the quartz-containing fractions of red and grey granite. CONCLUSIONS The cancer morbidity and mortality figures of the three different granite areas, combined with the found differences in biological activity of granite dusts and a hypothesis that there is a cancer-inducing mechanism for ROS, point to a direct role for quartz in cancer induction.