Human immunodeficiency virus-I (HIV-1) infected patients show a gradual loss of natural killer (NK) cells that correlates with disease progression. However, the effect of HIV-1 infection on different NK cell subsets has not been fully characterized. In healthy individuals most NK cells are CD3-CD56+ and two different subpopulations, CD56(dim) and CD56(bright), can be distinguished by the mean fluorescence intensity. Although it was originally suggested that CD56(bright) NK cells represent the precursors of the CD56dim subpopulation, recent cumulative data indicate that CD56(bright) and CD56(dim) NK cells are phenotypically, functionally, and developmentally different NK cell subsets. In this study, the analysis of CD56(bright) and CD56(dim) NK subsets showed that neither the number nor the phenotype of CD56(bright) NK cells were significantly altered in treatment-naive HIV-1-infected individuals, whereas the number of CD56(dim) NK cells was decreased. We also have studied NK cell subsets defined by the expression of CD56 in combination with CD16, CD161, or CD94 molecules. Our results demonstrated a preferential decrease of CD3-CD56+ NK cells coexpressing CD16 and CD161 but lacking CD94 molecules. On the contrary an increased percentage of NK cells that do not express CD56 molecules but express CD16, CD161, or CD94 was also found in HIV-1-infected individuals. As it has been proposed that these CD56-negative NK cells expressing other NK cell receptors represent immature NK cells with low cytolytic capacity, our results support that a defective differentiation from immature CD56 negative NK cells to mature CD56(dim) NK cells occurs in HIV-1 infection.