The efficacy and toxicity of recombinant human granulocyte colony-stimulating factor (rh G-CSF, KRN8601) given subcutaneously was evaluated in patients with advanced lung cancer undergoing intensive chemotherapy. Twenty-nine and 30 patients with or without prior therapy were enrolled in this study. At dose levels of 50, 90 and 130 micrograms/m2 of rh G-CSF for 14 consecutive days after chemotherapy, the mean neutrophil nadir counts, the mean neutrophil nadir ratios and the duration of neutropenia (days of less than 1000/mm3) were significantly improved. No significant differences were seen in frequency and duration of febrile episodes (greater than 38 degrees C). When rh G-CSF is given subcutaneously, the dose required for an equal effect in alleviating neutropenia is 50% of that required when it is given intravenously. The monocyte counts in the peripheral blood were also significantly increased after chemotherapy cycles with rh G-CSF. The cumulative plasma concentration of rh G-CSF showed a decrement after 7-9 days despite maintenance of the same dose of rh G-CSF for the entire 14 days. In conclusion, 50-130 micrograms/m2 of sc rh G-CSF increased the neutrophil nadir count and shortened the duration of neutropenia in patients undergoing intensive chemotherapy for lung cancer without intolerable side effects.