The vitamin D2 content of white button mushrooms is relatively low. UV exposure produces vitamin D2 by rapid conversion of ergosterol to ergocalciferol. Commercial-scale UV treatment has been used to produce vitamin D-enhanced mushrooms. The reliability of a consumer-friendly protocol to increase vitamin D2 in mushrooms by a nutritionally meaningful amount using exposure to sunlight was evaluated. Sliced white button mushrooms were exposed to sunlight for 15, 30, or 60 minutes in 16 experiments at different times of day, seasons, and cloud cover. Vitamin D2 was measured by HPLC with 3H-vitamin D3 internal standard. Change in vitamin D2 per 70 g serving relative to untreated mushrooms was evaluated. Vitamin D2 in all unexposed mushrooms was <30 IU/70 g (<5% DRI) (median, <7 IU/70 g). Regardless of season, treatment for 15 minutes between 9:30 a.m. and 3:30 p.m. under partly cloudy to clear conditions increased vitamin D2 by157-754 IU/70 g (26-126%DRI), and up to 1142 IU/70 g total increase was observed after 30 min. On overcast and mostly cloudy days the gain was 76-178 IU/70 g (13-30%DRI) after 15 minutes, but after one hour the level was comparable to 15 minutes of treatment in clear conditions. Trials by consumers at four different geographic locations resulted in increases of 367-905 IU/70 g. A preliminary trial showed dramatically elevated vitamin D2 contents in other mushroom types exposed 15 minutes under clear conditions. These results demonstrate that vitamin D2 in mushrooms can be reliably enhanced by at least 25% of the DRI (150 IU; 3.75 μg)/70 g serving by exposure to sunlight for as little as 15 minutes on a clear or partly cloudy day between 9:30 a.m. and 3:30 p.m., and >100% (>600 IU) in many cases. Even under conditions of lower UV intensity similar increases can be achieved after 30-60 minutes.