Pilocytic astrocytoma is a slow-growing, circumscribed glioma that most frequently occurs within the pediatric population. In general, surgical resection for pilocytic astrocytoma is thought to be curative with tumor recurrence or malignant transformation being relatively rare. However, there have been very few studies specifically looking at the prognosis for adult patients diagnosed with pilocytic astrocytoma. To evaluate the frequency of recurrence and malignant transformation of pilocytic astrocytoma in adults, we performed a retrospective analysis of all adult patients who underwent surgical resection for this tumor at our institution over a period of 10 years. In our cohort of 20 patients, there were 6 (30%) recurrences with four patients requiring repeat surgery due to symptomatic progression. Relatively rapid recurrences were noted with the median time to recurrence being 16.5 months. All recurrences occurred within 4 years of initial surgery while patients requiring repeat surgery presented within 17 months of initial surgery. Based on this study estimated rates of freedom from recurrence (FFR) at 12 and 24 months after initial surgery are 94 +/- 5% and 76 +/- 10%, respectively. A high rate of malignant transformation was observed in the patients that underwent repeat surgery with 75% (3/4) progressing to anaplastic astrocytoma on pathological examination. This study provides further evidence that the clinical course of a subset of adult patients with pilocytic astrocytoma will not be benign. The potential for rapid tumor recurrence and malignant transformation necessitates careful post-operative follow-up for adult patients with this tumor.