There is uncertainty about the amount of sun exposure required to increase low blood 25-hydroxyvitamin D (25(OH)D3) levels, a possible disease risk factor. The study aimed to quantify the association between sun exposure and serum 25(OH)D3 concentrations in a multiethnic community sample (n=502) living in Auckland (37°S) and Dunedin (46°S), New Zealand, aged 18-85 years. They wore electronic ultraviolet dosimeters between March and November (autumn, winter and spring) for 8 weeks to record their sun exposure. This was converted to standard erythemal doses (SEDs), corrected for clothing to generate equivalent full-body exposures, SEDEFB. Blood samples were collected at the end of weeks 4 and 8 to measure 25(OH)D3. Median weekly SEDEFB was 0.33 during weeks 1-4 and 0.34 during weeks 5-8. Weekly exposures <0.5 SEDEFB during weeks 5-8 were associated with decreasing 25(OH)D3 concentrations at the end of week 8. There was a non-linear association between sun exposure and 25(OH)D3, with most of the increase in 25(OH)D3 being at exposures <2 SEDEFB per week. This finding suggests that vitamin D status is increased by regular small sun exposures (<2 SEDEFB per week), and that greater exposures result in only small additional increases in 25(OH)D3.