1. The exposure of Asian immigrants to effective quantities of naturally occurring ultraviolet light was investigated by observing the increase in serum 25-hydroxyvitamin D concentrations during the summer months. 2. The seasonal increase in adult males was greater than in adult females. 3. There was a significant positive correlation between the seasonal increase and the prevailing 25-hydroxyvitamin D concentration, compatible with the view that the accumulation of inadequate stores of 25-hydroxyvitamin D during the summer months results in the low serum concentrations seen in Asian immigrants.