A risk model for febrile neutropenia (FN) after conventional cytotoxic chemotherapy, based on early (day 5) lymphopenia and the dose of chemotherapy, has been described. A risk index based on parameters available at day 1 would be easier in daily practice. The objectives of this work were (1) to investigate a risk model for FN using only day 1 blood cell count and (2) to compare the day 1 and day 5 risk models. Three series of patients were used for the delineation and/or validation of these two risk models: (1) the exhaustive cohort of 950 patients treated in the Department of Medicine of the CLB in 1996 (CLB-1996 series), (2) the Elypse 1 series, a prospective series of 321 patients treated in community hospitals and regional cancer centres, and (3) a previously reported Elypse 0 series of 329 patients. Day 1 blood cell count was available in all three series, while day 5 blood cell count was available only in the Elypse 0 and 1 series. In the CLB-1996 series, 92 (9.7%) patients experienced FN; only chemotherapy dose and day 1 lymphopenia < or =700 microl(-1) had an independent prognostic value for FN in multivariate analysis. In patients with both risk factors ('high-risk group'), the incidence of FN was 44, 50 and 61% in the CLB-1996. Elypse 1 and 0 series, respectively, indicating that the 'day 1' risk model enables one to identify patients at high-risk for FN. Besides, the observed incidence of FN in the high-risk group of the 'day 5' model (i.e. patients with day 5 lymphopenia < or =700 microl(-1) and receiving high-risk CT) was 45 and 69% in the Elypse 0 and 1 series, respectively. In the Elypse 1 and 0 series, 15 and 12% of all patients who experienced FN were in the high-risk group of the 'day 1' risk model as compared to 25 and 62% for the high-risk group of the 'day 5' risk model. Both day 1 and day 5 lymphopenia are associated with an increased risk of FN in patients treated with chemotherapy. The 'day 1' model identifies a small population of patients at high risk for FN, but has a lower sensitivity than the day 5 model.