Cytotoxicity mediated by natural killer (NK) and lymphokine-activated killer (LAK) cells may be of significance in host defense against viral infections. This study included 347 patients infected with human immunodeficiency syndrome virus (HIV) type 1 and 110 controls. The NK cell activity, either unstimulated or stimulated with interferon-α (IFN-α) or interleukin-2 (IL-2), and the LAK cell activity were suppressed in patients, but the NK/LAK cell activity did not differ between patients with AIDS and patients without AIDS. However, the IFN-ol-stimulated NK cell activity and LAK cell activity were reduced in patients with symptoms of HIV disease (CDCIV) when compared with asymptomatic patients (CDCII + III). When the data were analyzed by multiple linear regression, the percentage of CD4⁺ cells had a positive effect on these two parameters in patients without AIDS, whereas the percentage of CD4⁺ cells had no significant effect on unstimulated and IL-2-stimulated NK cell activity in these patients. In controis and AIDS patients, the percentage of CD4⁺ cells had no effect on NK/LAK cell activity in multiple linear models. The total number of CD16⁺ cells was low in patients compared to controls, whereas the percentages of CD16⁺, CD56⁺, and CD16⁺ CD56⁺ were either normal or elevated. Therefore, the decrease in NK cell subpopulations did not contribute to the observed depression in NK/LAK cell activity in vitro. It is concluded that natural immunity is suppressed in HIV-seropositive patients primarily because of a qualitative defect of the NK/LAK cells. This qualitative defect includes a reduced responsiveness to IFN-α, which is progressive until the onset of symptoms, and possibly related to the loss of CD4⁺ cells.