In the Phase I portion of the study, 20 patients per site each receive isotretinoin in escalating doses. If a patient experiences grade 2 or worse toxicity (or grade 3 or worse hypertriglyceridemia), dose is reduced to the previously tolerated dose for the remainder of the 6 week period. Patients are then reassessed for anal neoplasia; those with no progression and no grade 2 or worse toxicity receive an additional 6 weeks of isotretinoin in combination with interferon alfa-2a. For Phase II of the study, a separate group of patients who have undergone ablative therapy are randomized to one of three arms (26 patients/arm): isotretinoin alone at the dose tolerated by at least 60 percent of patients in Phase I; isotretinoin plus interferon alfa-2a; or observation only. Treatment continues for 48 weeks.