To characterize the relationship between occupational sun exposure and seasonal variations in serum 25-OH-D3, four consecutive measurements of 25-OH-D3, one per season, were taken in 122 outdoor and 104 indoor Israeli workers. Continuous UVB measurements, taken in Beer Sheva, Israel, provided the average daily standard erythema dose (SED) of ambient solar UVB. The average daily exposure of the outdoor and indoor workers to solar UVB was 4.4+/-1.6 h (4.0-37.6 SED) and 0.9+/-0.5 h (0.6-8.2 SED), respectively. At each season mean 25-OH-D3 were significantly higher among outdoor workers than among indoor workers. Mean 25-OH-D3 increased significantly from spring to autumn in both gender and occupational groups. Adjusting for confounders, high (>median) 25-OH-D3 among males was significantly associated with occupational sun exposure in the autumn (odds ratio [OR] 4.31; 95% confidence interval [CI] 1.4-13.3), and among females in the spring (OR 3.35; 95% CI 1.53-7.32). Among this working population optimal vitamin D status (>or=30 ng mL(-1)) was approached only in summer by males working either outdoor or indoor. In the rest of the year 25-OH-D3 ranged between >or=20.0 and 29.0 ng mL(-1). Monitoring 25-OH-D3 may disclose undesirable vitamin D status following reduced sun exposure for skin cancer prevention among outdoor workers.