The nucleoside analogue, pentostatin, has demonstrated high complete response rates and long relapse-free survival times in patients with hairy cell leukemia, a disease that historically had been unresponsive to treatment. Long-term data on duration of overall survival and relapse-free survival and incidence of subsequent malignancies with this agent are lacking. Patients completing the treatment phase of a randomized, intergroup study who received pentostatin as an initial treatment or who crossed over after failure of interferon alpha were followed for survival, relapse, and diagnosis of subsequent malignancies. Two hundred forty-one patients treated with pentostatin as initial therapy (n = 154) or who crossed over after failure of interferon alpha (n = 87) were followed for a median duration of 9.3 years. Estimated 5- and 10-year survival rates (95% confidence interval) for all patients combined were 90% (87%-94%) and 81% (75%-86%), respectively. In the 173 patients with a confirmed complete response to pentostatin treatment, 5- and 10-year relapse-free survival rates were 85% (80%-91%) and 67% (58%-76%), respectively. Survival curves for patients initially treated with pentostatin and those crossed over were similar. Only 2 of 40 deaths were attributed to hairy cell leukemia. The mortality rate and incidence of subsequent malignancies were not higher than expected in the general population. Pentostatin is a highly effective regimen for hairy cell leukemia that produces durable complete responses. Subsequent malignancies do not appear to be increased with pentostatin treatment.