The laboratory findings in patients receiving high-dose vitamin A as adjuvant treatment for stage I lung cancer are here reported. A group of 283 patients were randomized to either treatment with retinyl palmitate (300,000 IU daily for 12 months) or standard follow-up, and are now evaluable after a median observation period of 28 months. At regular intervals, all the patients underwent a physical examination, chest roentgenogram, blood chemistries, haematological assays, hepatic and renal function tests and determinations of serum triglycerides and cholesterol. Serum transaminase abnormalities were of similar magnitude in cases and controls, while gamma-glutamyltransferase levels were abnormally elevated in 69% of the treated patients compared to 39% of controls (mean values 149 vs 57 IU/l at 24 months, P less than 0.05). Serum triglyceride concentrations over 150 mg/dl were seen in 74% of treated patients compared to 43% of controls at 12 months, the average concentration was 283 mg/dl compared to 179 mg/dl (P less than 0.05). Cholesterol levels showed a modest, non-significant rise with time in both groups, and there was no other laboratory evidence of toxicity attributable to vitamin A. Serum retinol and retinol-binding protein, assessed on a limited sample of patients, were higher in the treatment arm (P less than 0.05) at 12 months. In our experience 300,000 IU/day of retinyl palmitate can be administered as a possible chemopreventive agent with reasonable safety for up to 2 years.