Fifty-seven patients with metastatic lung carcinoma were treated with either high-dose cyclophosphamide (Cy) alone or with a combination of high dose Cy and prednisolone (Pred) 100 mg/m2 orally daily for two days. The Cy was given IV on three occasions, at 1.5 g/m2, 2.5 g/m2 and 3.5 g/m2 with three-week intervals between courses. The overall response rate was 57% (18% CR), a median survival of 24 weeks (range, 6--130) for Cy alone, and 24% (3% CR), a median of 14 weeks (range, 1--94) for Cy + Pred. Patients with small cell carcinoma given Cy alone had a 69% response rate (19% CR), a median survival of seven months, and with non-small cell pathologic conditions 42% (16% CR), a median survival of 16 weeks. Performance scores and survival were better for responding patients. Addition of Pred did not improve the therapeutic efficacy of high dose Cy or ameliorate toxicity. No marked or unexpected toxicity was observed with the high-dose Cy regimen. The blood counts had returned to normal by three weeks in the great majority of patients. A short course of high-dose Cy was not associated with unacceptable side effects and the therapeutic results obtained were superior to those described for Cy at conventional dosage. High-dose Cy is of value to patients with metastatic lung cancer, and the incorporation of the regimen into chemotherapeutic combinations could be advantageous.