Earlier studies reported the associations among testosterone hormone, autoimmunity, and left-handedness. In the present study, sex differences in tuberculin reaction, a measure of cell-mediated hypersensitivity, serum free and total testosterone levels in controls and patients with autoimmune diseases were studied. There was a sex difference in right and left tuberculin reactions in controls, but not in patients. Both right and left tuberculin reactions were smaller in male and female patients than male and female controls. Free and total testosterone levels were higher in male controls than in male patients. Total testosterone levels were higher in female controls than in female patients. These results suggest that autoimmune diseases may be associated with a decrease in the blood testosterone concentrations.