Natural killer (NK) cells in HIV‐infected patients have a reduced ability to generate non‐MHC restricted cytotoxicity to a variety of target cells. The authors investigated antibodies to NK cells in HIV‐infected patients and evaluated effects of these antibodies to NK cell numbers and function. Antibodies to NK cells were determined in 160 HIV‐infected patients and 35 healthy controls. Flow cytometric whole blood methods were developed to detect antibodies to NK cells. Antibodies to asialo‐GM1 were detected by TLC immunostaining. The presence of antibodies to NK cells was demonstrated in plasma of about one‐third (54/160) of HIV‐infected patients but rarely in controls (2/35). Autoantibodies bound to NK cells in vivo and were detected by a strong increase of surface immunoglobulin (Ig) on NK cells of HIV‐infected patients. Anti‐NK cell antibodies were warmreactive antibodies rather of IgG than of IgM phenotype. The prevalence of specific antibodies to asialo‐GM1 was low (12.5%). Numbers of circulating NK cells did not differ significantly between antibody positive (99.5/μl) and antibody negative (141/μl) patients (P = 0.3). However, pre‐incubation of healthy donors' NK cells with autoantibody positive plasma significantly inhibited cytotoxicity to K562 leukaemic cells (P = 0.002). Autoantibodies to NK cells in HIV‐infected patients are present in the plasma of one‐third of HIV‐infected patients and are bound to NK cells in vivo. There is evidence that these autoantibodies can induce NK cell defects similar to those seen in vivo