The treatment costs for human immunodeficiency virus (HIV)-infected patients continue to rise as patients survive longer because of advances in antiretroviral therapy and effective chemoprophylaxis. Medication costs per patient increase in proportion to progressive immunodeficiency. We retrospectively studied medication costs for 196 HIV-infected patients with stratification by CD4-lymphocyte count. Medication costs per patient-month were correlated with CD4-lymphocyte count (linear regression, r = -.53, P < 0.01), with higher costs associated with lower CD4 counts. The medication cost for patients with CD4 counts < 100 cells/mm3 averaged $1043 per month. Medication costs per patient increase with the development of each new opportunistic infection or other AIDS-associated condition. Costs can be expected to increase as new therapeutic agents are introduced, as treatment is initiated at earlier stages of HIV infection, and as more patients survive to the point of severe CD4-lymphocyte depletion.