Neutropenia is a major adverse effect of cancer chemotherapy and sometimes causes life-threatening events. The present study was therefore conducted to identify risk factors for such neutropenia. Forty patients who had received chemotherapy at 3- or 4-week intervals for advanced lung cancer from May 1991 through February 1997 were analyzed retrospectively. Thirty-seven of the patients had received cisplatin-based chemotherapy. The mean neutrophil count on days 6 to 8 in 32 patients who developed grade 3 or 4 neutropenia during chemotherapy was not significantly different from that in eight patients who developed grade 1 or 2 neutropenia during chemotherapy. However, the mean leukocyte and monocyte counts on days 6 to 8 in the 32 patients with grade 3 or 4 neutropenia (5,181 +/- 1,830/microl and 87 +/- 84/microl, respectively) were significantly lower than those in the eight patients with grade 1 or 2 neutropenia (7175 +/- 1671/microl and 248 +/- 127/microl, respectively; p = 0.008 and p = 0.0001). Moreover, all 30 patients with a monocyte count of less than 150/microl on days 6 to 8 had grade 3 or 4 neutropenia and 8 of 10 patients with a monocyte count of 150/microl or higher on days 6 to 8 had grade 1 or 2 neutropenia, despite the absence of a correlation between the leukocyte count on days 6 to 8 and the neutrophil nadir. We conclude that a monocyte count of less than 150/microl on days 6 to 8 may be a predictor of grade 3 or 4 neutropenia during cancer chemotherapy at 3- or 4-week intervals (sensitivity 94%, specificity 100%).