Cyclophospamide was given in two dose schedules to 25 patients with a variety of nonlymphoid solid tumors. Eleven patients were given 18 courses of cyclophosphamide at a total dose of 60 mg/kg. Sixteen patients received 26 courses at a total dose of 100 mg/kg. Two patients were treated with both regimens. Partial responses were achieved in two patients treated with 60-mg/kg dose of cyclophosphamide. One of these patients had osteogenic sarcoma and the other had renal carcinoma. The higher dose also produced two partial responses, one in a patient with anaplastic carcinom a of the lung and the other in a patient with anaplastic carcinoma of the lung and the other in a patient with embryonal testicular carcinoma. Mean leukocyte counts fell to a nadir of 1400 cells/mm after 60 mg/kg while they dropped to below 1000 cells/mm for 5 days after 100 mg/kg of cyclophosphamide. Mean platelet counts remained above 150,000 platelets/mm after both cyclophosphamide schedules. In fective complications were documented aftter three of the 18 courses at 60 mg/kg and after ten of the 26 courses at 100 mg5kg. In the latter group, there were three episodes of bacteremia, including one death from pseudomonas sepsis. Nonhematologic toxicity noted with the 100-mg/kg dose of cyclophosphamide included rare instances of electrocardiogram changes and serum enzyme alterations compatible with myocardial toxicity. The intensive cyclophosphamide therapy did not appear to result in an increased antitumor response in malignancies usually considered to be refractory to alkylating agents.