Peripheral blood cells with the HNK-1 phenotype were studied in HIV infected patients of which 28 with Asymptomatic Infection (AI), 64 with Persistent Generalized Lymphoadenopathy (PGL), 9 with AIDS Related Complex (ARC), and 12 with AIDS. Eight normal subjects served as controls. Two-color immunofluorescence by flow cytometry analysis showed in all of them a significant increase of the mean percentage of HNK+T3- lymphocytes (greater than 20%) as compared to controls (6%). Only in AI the mean absolute count was significantly higher (776/cmm) than control's one (152/cmm). Percentages and absolute counts of HNK+T3- lymphocytes were similar to normal ones. In AI and PGL HNK+T3+ cells correlated directly with T8 lymphocytes and inversely with T4 cells and T4/T8 ratio. These results indirectly suggests that HNK+T3+ cells represent a subset of suppressor/cytotoxic lymphocytes. The results in ARC and AIDS were somewhat equivocal and deserve further study in larger samples. No correlation was found between HNK+T3+ and HNK+T3- cells. The expansion of HNK+T3+ cells was parallel to that one of T8 lymphocytes expressing CD8 antigen at high surface density which were previously reported as having cytotoxic activity. Follow-up studies of the HNK cell peripheral pattern will clarify if it can be regarded as an early predictor of clinical outcome.