We identified 10 individuals who had undiagnosed human immunodeficiency virus type 1 (HIV-1) infection at the time of smallpox vaccination. Mean CD4 cell count was 483 cells/mm3 (range, 286-751 cells/mm3), and mean log10 plasma HIV-1 RNA load was 4.13 copies/cm3 (range, 2.54-5.16 copies/cm3). All vaccinees (3 primary and 7 repeat) had a normal, robust reaction without complications. Smallpox vaccine was well-tolerated in this small series of HIV-1-infected military personnel.