The safety, efficacy, and tolerability of nelfinavir (NFV)-containing antiretroviral therapy were evaluated in patients coinfected with HIV and hepatitis C undergoing methadone maintenance at an urban outpatient opioid treatment program serving a minority adult population. Eligibility covered methadone-maintained patients coinfected with HIV and hepatitis C who had received or were currently receiving NFV. The yield was 51 case patients. Parameters examined looked into safety, efficacy, and tolerability. Nelfinavir was discontinued in 2 patients for liver function abnormalities but resumed in 1 patient. One patient developed laboratory abnormalities during NFV therapy that were not present before NFV therapy; in 12 case patients, pre-NFV therapy liver function abnormalities resolved completely during NFV therapy. There was a statistically significant increase in CD4 count during NFV therapy. Viral load decreased or was unchanged in 10 case patients and increased in 8, of whom 5 had a CD4 count increase during NFV therapy. Three patients had diarrhea and 4 patients had constipation. Nelfinavir was not discontinued -- neither was dose adjusted -- in any of these patients. Patients who had received NFV > or =36 months had a smaller increase in mean methadone dose as compared with patients who had received NFV <36 months. The results show that NFV is safe, efficacious, and well tolerated.