Experimental data support the suppressing effect of vitamin D on lung carcinogenesis, but epidemiologic evidence is limited. The aim of the present study was to evaluate whether serum 25-hydroxyvitamin D [25(OH)D] level is associated with the risk of lung cancer in a prospective cohort study in Finland. 25(OH)D levels were measured by RIA from serum collected at baseline (1978--1980) from 6,937 men and women. During a maximum follow-up of 24 years, 122 lung cancers were identified. After adjustment for potential confounders, no overall significant association between vitamin D and lung cancer risk was observed [relative risk (RR) for the highest versus lowest tertile, 0.72; 95% confidence interval (95% CI), 0.43-1.19; Ptrend = 0.22]. There was a statistically significant interaction between vitamin D and sex (P = 0.02) and age (P = 0.02): serum 25(OH)D level was inversely associated with lung cancer incidence for women (RR, 0.16; 95% CI, 0.04-0.59; Ptrend < 0.001) and younger participants (RR, 0.34; 95% CI, 0.13-0.90; Ptrend = 0.04) but not for men (RR, 1.03; 95% CI, 0.59-1.82; Ptrend = 0.81) or older individuals (RR, 0.92; 95% CI, 0.50-1.70; Ptrend = 0.79). In conclusion, although there was no overall association between vitamin D and lung cancer risk, women and young participants with a higher level of vitamin D were observed to have a lower lung cancer risk. Although experimental data support the suppressing effect of vitamin D on the development of lung cancer, large epidemiologic studies from different populations with repeated measurements of vitamin D are warranted to confirm this finding.