Background Alopecia areata (AA) is a common inflammatory disorder targeting the hair follicles leading to non-scaring hair loss. The pathogenesis of AA is still unclear, despite the accumulating evidence of the immune-mediated nature of this disease. So, in this study, we aimed to assess the level of CD4 T cells, CD8 T cells and natural killer (NK) cells in the peripheral blood of patients with active AA and control subjects, and to evaluate the association between the level of those cells and the duration of disease in different subtypes of AA. Results Fifty female patients and 50 age- and sex-matched healthy controls were enrolled in this case control study. CBC analysis and the level of CD4, CD8 T cells and NK cells were evaluated during the active stage of the disease. We found that CD4, CD8 T cells and NK cells proportion was significantly lower (P < 0.05) in patients with active stage AA compared with healthy subjects, however, the ratio of CD4:CD8 T cells was significantly higher in patients than control subjects. The level of CD4, CD8 T cells CD56 bright CD16− % NK cells were positively correlated with the disease duration. Conclusion Active stage of AA disease is associated with a reduction of the circulating CD4, CD8 T cells and NK cells and an increase in CD4/CD8 T cells ratio, however, the level of those cells were higher with prolonged disease duration. Our findings confirm that immune mechanisms are involved in the pathogenesis of AA.