The aim of this study was to determine the efficacy of oral beta-carotene supplementation for the correction of an oxidant-antioxidant imbalance in cystic fibrosis (CF). We studied 24-patients with cystic fibrosis and 14 healthy controls. 13 CF-patients were allocated to a CF-supplementation group, which received 1 mg beta-carotene/kg BW/d up to a body weight (BW) of 50 kg, patients with a BW greater 50 kg received 50 mg beta-carotene/d for 12 weeks. For the following 12 weeks all patients of the CF-supplementation group were treated with 10 mg beta-carotene/d. Placebos with starch were applied to 11 CF-patients. Baseline plasma beta-carotene concentrations of CF patients (mean +/- SD, 0.08 +/- 0.04 mumol/l) were significantly lower than those of age-matched controls (o.3 +/- 0.1 mumol/l) (p < 0.001). beta-carotene concentrations of the CF-supplementation group increased rapidly and reached a value of 0.6 mumol/l after 12 weeks of supplementation. Normal values were measured for plasma ascorbate and alpha-tocopherol. Plasma retinol concentrations were in the lower normal range and did not increase during supplementation. Total antioxidative capacity in plasma of the CF-supplementation group increased after 12 weeks of supplementation at an extent of 12%. Positive influence was indicated by a decrease of plasma malondialdehyde. Thus oral beta-carotene supplementation is effective in normalizing status of beta-carotene and malondialdehyde in CF patients.