Toxic effects of 5-fluorouracil (5-FU) and oxaliplatin (L-OHP), two active drugs against metastatic colorectal cancer, varied by 50% or more according to circadian dosing time in mice or rats. Adaptation of chemotherapy delivery to circadian rhythms (chronotherapy) was assessed in fully ambulatory outpatients, using multichannel programmable pumps. These devices allowed us to reliably test the clinical relevance of such a chronotherapy principle. First, single agent 5-day chronomodulated schedules were devised and assessed in Phase I and II trials with 5-fluorouracil (5-FU, peak delivery at 4:00 h) or oxaliplatin (L-OHP, peak at 16:00 h). Both schedules were then combined, folinic acid (FA) being added, synchronous with 5-FU infusion. This three-drug chronomodulated regimen (chrono-FFL) produced a 58% response rate (95% C.I.: 48-68%) in 93 patients with metastatic colorectal cancer, 46 of whom had previously received chemotherapy. In the first European randomised trial in 92 previously untreated patients, chronomodulated three-drug delivery achieved 53% response, as compared to 32% in those patients receiving flat infusion (P = 0.038). These respective figures were confirmed in a subsequent multicentre randomised trial involving 186 additional patients. Since the most active schedule was also the least toxic one by 2- to 10-fold, chrono-FFL was further intensified in three consecutive Phase II trials involving a total of 200 additional patients. Results suggest that both response rate and quality were further improved with such treatment intensification. Thus, chrono-FFL more than doubled the activity of chemotherapy against metastatic colorectal cancer in a multicentre European setting.(ABSTRACT TRUNCATED AT 250 WORDS)