Solar ultraviolet-B radiation (UVB) is essential for epidermal vitamin D production. We aimed to quantitate the relationship between personal solar UV exposure and serum 25hydroxy vitamin D (25[OH]D) concentration. Blood was collected for 25(OH)D analysis in 207 South Australian adults aged 27-61 years. At the time of blood collection, each participant completed a questionnaire, which included a calendar for recall of sun exposure in the preceding 16 weeks. We examined the association between solar UV exposure and serum 25(OH)D graphically from smoothed scatter plots, and modeled it using multiple linear regression, with age, sex and body mass index as covariates. Estimated erythemal solar UV exposure in the 6 weeks before blood collection best predicted serum 25(OH)D concentrations. Serum 25(OH)D rose with increasing personal solar UV exposure to a maximum of about 89 nmol L-1 at an estimated mean weekly solar erythemal UV exposure of about 1230 mJ cm-2. The maximum was the same after accounting for clothing coverage and was reached at an estimated whole body equivalent exposure to ambient UV of ca 700 mJ cm -2. These results suggest that an average maximum serum 25(OH)D of ca 89 nmol L-1 is achieved from sun exposure in a healthy Australian adult population. Sun exposure is the major source of vitamin D in most population. However, the amount of incidental sun exposure conducive to the production of serum vitamin D remains unclear. This study shows a quantitative link between recalled personal solar UV exposure and serum 25(OH)D. The relationship was curvilinear and reached a plateau at about 89 nmol L -1. The plateau level appeared to be higher in men than in women. Further understanding of the significance of these plateaus, which suggest physiological maxima, could contribute to future recommendations for fortification or supplementation with vitamin D.