Objective: To examine the feasibility of balancing sunlight exposure to meet vitamin D requirements with sun protection guidelines. Design and setting: We used standard erythemal dose and Ultraviolet Index (UVI) data for 1 June 1996 to 30 December 2005 for seven Australian cities to estimate duration of sun exposure required for fair-skinned individuals to synthesise 1000 IU (25 μg) of vitamin D, with 11% and 17% body exposure, for each season and hour of the day. Periods were classified according to whether the UVI was <3 or 3 (when sun protection measures are recommended), and whether required duration of exposure was 30 min, 31-60min, or >60min. Main outcome measure: Duration of sunlight exposure required to achieve 1000 IU of vitamin D synthesis. Results: Duration of sunlight exposure required to synthesise 1000IU of vitamin D varied by time of day, season and city. Although peak UVI periods are typically promoted as between 10 am and 3 pm, UVI was often 3 before 10am or after 3pm. When the UVI was <3, there were few opportunities to synthesise 1000IU of vitamin D within 30 min, with either 11% or 17% body exposure. Conclusion: There is a delicate line between balancing the beneficial effects of sunlight exposure while avoiding its damaging effects. Physiological and geographical factors may reduce vitamin D synthesis, and supplementation may be necessary to achieve adequate vitamin D status for individuals at risk of deficiency.