This study was conducted to characterize the recurrent aphthous ulcers (RAU) found in association with human immunodeficiency virus (HIV) infection, to examine evidence for increased severity of the ulcers with HIV disease, and to determine whether increased severity is associated with abnormalities of peripheral blood lymphocyte subsets. Seventy-five HIV-seropositive patients with RAU were followed for up to 2 years, and lymphocyte subsets were analyzed in 42. Minor, herpetiform, and major ulcer types were seen, but unexpectedly, 66% of the patients had the usually uncommon herpetiform and major types. These types were temporally associated with symptomatic HIV disease. Patients with major RAU were significantly more immunosuppressed than those with minor or herpetiform RAU in that they had fewer CD4 and CD8 lymphocytes (p less than 0.05). The lesion of RAU is considered to represent a local breakdown in immunoregulation. The systemic immune imbalance seen with HIV disease may amplify the local defect and lead to more severe ulcers.