Objectives. This study sought to identify and characterize seriously mentally ill patients with HIV infection. Methods. Medicaid beneficiaries with HIV/AIDS were identified through the merging of New Jersey HIV/AIDS Registry and Medicaid eligibility files. Claims histories were used to classify individuals as having schizophrenia, major affective disorder, or no serious mental illness. Results. Of 8294 individuals, 476 (5.7%) were classified as having schizophrenia, and 564 (6.8%) were classified as having major affective disorder. Those with serious mental illness were more likely than other groups to be injection drug users and to have claims indicative of substance abuse. Conclusions. Individuals with serious mental illness are a significant but little-recognized subgroup of those with HIV infection.