Background: Mushrooms are the only non-animal food source of vitamin D. Wild mushrooms have naturally high vitamin D2 content, and cultivated mushrooms produce vitamin D2 from ergosterol when exposed to supplementary UV-B during the post-harvest phase. Objectives: This study investigated the effects of providing supplementary UV-B during the growth phase on vitamin D2 formation and the interactions with growth of mushrooms, as compared to supplementary UV-B during the post-harvest phase or exposure to sunlight for both cultivated and wild mushrooms. Methods: Experiments were carried out with exposure to supplementary UV-B just prior to harvest in the range of 0-2,400 mJ cm-2. Mushrooms grew for 2 days with or without repeated UV-B exposure each day. Vitamin D2 and growth rate were determined. Some mushrooms were post-harvest treated by exposure at 200 mJ cm-2 supplementary UV-B or natural sunlight, prior to vitamin D2 determination. Results: The content of vitamin D2 was 0.2-164 Âµg 100 g-1 fresh weight (FW), and there was a linear relationship between UV-dose up to 1,000 mJ cm-2 and vitamin D2 content. The fast growth rate of the mushrooms diluted the vitamin D2 from 24 to 3 Âµg 100 g-1 within 2 days of exposure at 200 mJ cm-2. Following repeated UV-B exposure, vitamin D2 increased to 33 Âµg vitamin D2 100 g-1. Growth was unaffected by UV-B. Post-harvest exposure to supplementary UV-B resulted in a higher vitamin D2 content of 32 Âµg 100 g-1 compared to the 24 Âµg 100 g-1 obtained from exposure to UV-B during the growth phase. In contrast, wild and cultivated mushrooms with and without exposure to sunlight had vitamin D2 content in the range of 0.2-1.5 Âµg vitamin D2 100 g-1. Conclusions: This study showed that mushrooms with a well-defined content of vitamin D2 can be obtained by exposure to supplementary UV-B just prior to harvest.