The feasibility of on-site primary care services and their use by human immunodeficiency virus HIV-seropositive and seronegative injecting drug users within an outpatient methadone maintenance program are examined. A 16-month prospective study was conducted within an ongoing cohort study of HIV infection at a New York City methadone program with on-site primary care services. The study group consisted of 212 seropositive and 264 seronegative drug injectors. A computerized medical encounter data base, with frequencies of primary care visits and with diagnoses for each visit, was linked to the cohort study data base that contained information on patients' demographic characteristics, serologic status, and CD4+ T-lymphocyte counts. Eighty-one percent of the drug injectors in the study voluntarily used on-site primary care services in the methadone program. Those who were HIV-seropositive made more frequent visits than those who were seronegative (mean annual visits 8.6 versus 4.1, P < .001), which increased with declining CD4+ T-lymphocyte counts; 79 percent of those who were seropositive with CD4 counts of less than 200 cells per cubic millimeter received on-site zidovudine therapy or prophylaxis against Pneumocystis carinii pneumonia, or both. Common primary care diagnoses for patients seropositive for HIV included not only conditions specific to the human immunodeficiency virus but also bacterial pneumonia, tuberculosis, genitourinary infections, asthma, dermatologic disease, psychiatric illness, and complications of substance abuse; those who were seronegative were most frequently seen for upper respiratory infection, psychiatric illness, complications of substance abuse, musculoskeletal disease, hypertension, asthma, and diabetes mellitus. Vaginitis and cervicitis,other gynecologic diseases, and pregnancy were frequent primary care diagnoses among both seropositive and seronegative women.