We prospectively evaluated the clinical and biochemical responses to enzyme-replacement therapy (ERT) with macrophage-targeted glucocerebrosidase (Ceredase) infusions in 5 patients (age, 3.5-8.5 years) with type 3 Gaucher's disease. The patients were followed for up to 5 years. Enzyme dosage ranged from 120 to 480 U/kg of body weight/month. Systemic manifestations of the disease regressed in all patients. Neurological deficits remained stable in 3 patients and slightly improved in 1. One patient developed myoclonic encephalopathy. Cognitive deterioration occurred in 1 patient and electroencephalographic deterioration in 2. Sequential cerebrospinal fluid (CSF) samples were obtained during the first 3 years of treatment in 3 patients and were analyzed for biochemical markers of disease burden. Glucocerebroside and psychosine levels were not elevated in these specimens, whereas chitotriosidase and quinolinic acid were elevated in 2 patients. Progressive decrease in the CSF levels of these latter macrophage markers during 3 years of treatment implies a decreased number of Gaucher cells in the cerebral perivascular space. Similar changes were not observed in the patient who had a poor neurological outcome. In conclusion, ERT reverses systemic manifestations of type 3 Gaucher's disease and appears to reduce the burden of Gaucher cells in the brain-CSF compartment in some patients.