Natural killer (NK) cells were enumerated by three-color immunofluorescence in 255 uninfected and 399 human immunodeficiency virus-infected adults. Several dramatic alterations were observed. First, the median number and percentage of CD16+CD56+ NK cells, the subset that comprises > 90% of the NK cells in healthy adults, were severely decreased (median, 175/mm3 in uninfected controls; 63/mm3 in HIV-infected non-AIDS subjects). Even subjects with > 800 CD4+ cells/mm3 had decreased CD16+CD56+ NK cell levels (97/mm3). Second, the number of CD16+CD56- cells, an NK population that is rare in healthy adults, was elevated (median, 20/mm3 in uninfected controls; 64/mm3 in HIV-seropositive non-AIDS subjects). Third, the expression of CD16 on the NK cells was markedly reduced; some CD56+ cells and virtually all CD56- cells were CD16dim. Fourth, fluorescence-activated cell-sorting studies revealed little NK- or antibody-dependent cellular cytotoxic activity in the CD16dimCD56- cell population. These results indicate that the pathogenesis of HIV disease includes numerical alterations in subpopulations of NK cells. A better understanding of how HIV infection causes this aspect of pathogenesis is needed.