Twenty-four patients, including two with aplastic anemia and 22 with malignancy, underwent marrow transplantation after preparation with mechlorethamine, 0.3 to 2.0 mg/kg body weight. Fourteen of the 21 neurologically evaluable recipients developed immediate neurotoxicity a median of 4 days after treatment (range, 0 to 34 days). Confusion and disorientation were observed in six patients, headache in six, hallucinations n four, lethargy in four, tremors in three, paraplegia in one, seizure in one, and vertigo in one. Whereas acute symptoms cleared in 11 patients, three remained symptomatic until death. Twelve evaluable patients survived more than 60 days; all six with previous acute toxicity subsequently developed delayed onset of new neurologic findings (personality change, confusion, seizure, diplopia, or dementia) a median of 169 days (range, 70 to 248 days) after treatment. Cerebrospinal fluid analysis was usually normal but cerebral computed tomographic scans showed ventricular enlargement and electroencephalograms showed diffuse slowing. Postmortem histologic examination of brain showed neuronal degenerative changes with increased vascularity, gliosis, and perivascular fibrosis. Neurotoxicity appeared to increase with age and mechlorethamine dose and was commoner in patients given additional procarbazine or cyclophosphamide.