It is a common belief that sleep deprivation increases the susceptibility to diseases. In order to evaluate the effects of sleep deprivation on immune profile in humans, peripheral venous blood was obtained from sixteen healthy young male volunteers. Ten of the volunteers underwent 48 hours of sleep deprivation and the other six maintained their regular sleep schedule and acted as controls. The first blood samples were taken at the end of the first polysomnographic recording at 8:00 a.m. After this sampling, ten subjects were sleep deprived for 48 hours in sedentary conditions. The second and third blood samples were taken at the 24th and 48th hours. The subjects were recorded again to verify rebound effects of sleep deprivation after the third blood sampling. In this second polysomnographic recording, all sleep-deprived subjects showed slow wave and REM sleep rebound. The last blood samples were taken at the 72nd hour of study at 8:00 a.m. CD4, CD8, CD5, CD16, CD19 surface antigen positive lymphocyte subsets, serum IgG, IgM, and cortisol levels were assessed in all samples. Our results showed that the proportion of NK cells were decreased during sleep deprivation and returned to normal values after recovery sleep. In the control group, we did not observe any changes in the same direction as the sleep-deprived group.