Thirty-six cases of pilocytic astrocytomas treated at the Childrens Hospital of Los Angeles from 1984 through 1995 were reviewed. The mean age at initial presentation was 8 years (range 15 months to 14 years). These patients were followed for an average of 5.5 years. No patient was given chemotherapy or radiation therapy after the initial surgery for pilocytic astrocytoma. Twenty-three patients (64%) had a gross total resection with no residual tumor on immediate postoperative imaging studies. Three of these children had tumor recurrences 2-5 years after their initial surgery, requiring re-excision of their tumors. All 3 patients are subsequently tumor-free, with follow-up ranging from 4 to 10 years. In 4 patients with residual tumor involving the brainstem, there has been neither imaging evidence of tumor enlargement nor progression of clinical findings at 2.5, 4, 6 and 6 years, respectively. Nine of the 13 patients with residual tumor underwent re-excision, either for progression of symptoms or documented tumor growth on imaging studies. The second operation was undertaken an average of 1 year (range 1 month to 6 years) after the first. In 4 of these children (11% of our whole series), the recurrent tumor was classified as anaplastic astrocytoma. Three of these 4 children received radiation and/or chemotherapy, with only 1 patient showing disease progression in the follow-up period. Repeat blinded histopathological examination of these tumors confirmed both diagnoses. However, it was noted that 3 of the 4 pilocytic astrocytomas which subsequently showed anaplastic change initially displayed increased perivascular cellularity, while only 2 of the remaining 32 tumors exhibited this feature. These results encourage continued vigilance in the follow-up of pilocytic astrocytomas, and describe a histological feature which might indicate a more aggressive disease course.