A Phase I trial of interferon-alpha (IFN-alpha) administered at circadian-rhythm modulated rate was carried out to evaluate maximum tolerated dose (MTD) and toxicity in patients with advanced malignancies. Recombinant IFN-alpha-2b was administered as a 7-day continuous venous infusion with maximum delivery between 6 p.m. and 3 a.m. Entry dose levels were 0.2, 2 and 4 MU/m2/day. The dose was escalated by an amount equal to the starting dose until a maximum of 6 entry dose levels was achieved, with a 1-week rest between each cycle. The maximal daily dose of IFN-alpha administered was 24 MU/m2/day. A programmable-in-time ambulatory pump was used, so that all patients could receive their treatment at home. Eighteen patients were entered in the study and 16 were evaluable for toxicity. Toxicity was mild to moderate except for two patients who developed WHO grade III toxicity. No significant decline in performance status was associated with treatment. Two objective responses were observed in patients with previously treated metastatic renal cell carcinoma. As compared to standard s.c./i.m. administration or continuous nonchronomodulated i.v. infusion of IFN-alpha, this circadian schedule has allowed to deliver high doses of drug with acceptable toxicity.