Vitamin D is an effective treatment for prostate cancer in some patients, a UK study suggests.
A once daily dose reduced PSA level - an indicator of severity of disease - by as much as half in 20% of patients.
There has been much interest in vitamin D in prostate cancer after studies linking risk of the disease to sunlight exposure, the researchers said.
One expert agreed the findings were encouraging but said it needed to be tested in a bigger population.
The trial - results of which are due to be published in the journal BJU International - was set up after one patient got better when his wife bought him some vitamin D tablets.
Professor Jonathan Waxman, said the example had prompted him to assess the effects in a wider group of patients.
Out of 26 men with recurrent prostate cancer, who took a daily dose of vitamin D2 bought from the chemist, five responded to the treatment.
In two the PSA level, fell by more than half, in two by 25-50% and in one man it fell by less than 25%.
The effects in one man were sustained for 36 months.