The incidence of tuberculosis is very high in patients treated by hemodialysis particularly in the early stage of hemodialysis. The diagnosis of tuberculosis in dialysis patients was obscured as symptoms were nonspecific and extrapulmonary involvement was seen frequently. We investigated cell mediated immunity in dialysis patients in relation to the period of dialysis. The data indicate that dialysis patients show the following immunological impairments; 1) lymphopenia, 2) decreased B cell, 3) alteration of T cell subset, 4) decreased reaction of PPD skin test, 5) decreased T cell activity, 6) decreased IL-2 production, 7) decreased PHA induced lymphocyte blastogenesis, 8) decreased NK cell. Decreased immunologic host defence mentioned above may contribute to the high incidence of tuberculosis in the early stage as well as in the maintenance phase of dialysis.