Previous studies about the serum levels of vitamin D metabolites in epileptic patients have given conflicting results. We have investigated the influence of chronic anti-epileptic treatment on mineral metabolism in 17 ambulatory epileptic children who were studied for 2 seasons with high and low levels of solar radiation, respectively. No differences in serum calcium, phosphate or 1.25-dihydroxyvitamin D were observed between patients and control children. Patients also had normal levels of 25-hydroxyvitamin D [25(OH)D] in summer. However, serum 25(OH)D concentrations were lower in patients than in controls in winter months (12.6 +/- 1.4 versus 19.6 +/- 1.2 ng/ml, P less than 0.001). These findings point out the influence of the intensity of solar irradiation, and subsequently of vitamin D availability, on the effect of anticonvulsant drugs on vitamin D metabolism, and may help to explain the conflicting results of previous reports. Prophylactic vitamin D therapy should be considered when climatic conditions or patients' life styles do not allow an adequate exposure to sunlight.