An unusual neurological syndrome occurred in 4 of 158 patients treated for osteogenic sarcoma with combination chemotherapy. There was an abrupt onset of focal cerebral deficits approximately ten days after chemotherapy with vincristine and high-dose methotrexate plus citrovorum factor rescue. The syndrome was short lived and always occurred early in the course of treatment. Prolonged neurological deficits remained in 2 patients. When similar chemotherapy was reinstituted in the 4 patients, no further neurological complications ensued. Possible causes include a leukoencephalopathy related to methotrexate or an embolic cerebral vasculopathy related to necrotic tumor microemboli emanating from the lungs.