Serum levels of vitamin D metabolites were determined in 11 patients treated for cystic acne with a four-month course of isotretinoin (Roaccutane). The levels were measured before treatment and after two months of medication. We found a significant fall in the level of 1,25-dihydroxyvitamin D (p less than 0.01) and a significant increase in the molar ratio of 24, 25-dihydroxyvitamin D to 25-hydroxyvitamin D (p less than 0.05). No significant changes were found for the vitamin D metabolites 25-hydroxyvitamin D or 24,25-dihydroxy-vitamin D, for serum calcium, phosphorus, alkaline phosphatase or parathyroid hormone. Our data indicate early changes in the metabolism of vitamin D in patients on retinoid treatment.