Abstract.Cell‐mediated immunity (CMI) was studied in a group of 48 children with urinary tract infections (UTI) using a whole blood micromethod for lymphocyte stimulation in vitro. The patients were subdivided into pyelonephritis group (27 cases) and lower urinary tract infection (LUTI) group (21 cases) on the basis of fever, erythrocyte sedimentation rate, C‐reactive protein and renal concentration capacity. At the acute stage of infection the lymphocyte responsiveness to leucoagglutinin (LA) and concanavalin A (Con A) was suppressed in both groups, but the suppression was much greater in those with pyelonephritis. By 6 weeks after infection the lymphocyte responses were normal in most but not all cases. We conclude that an acute pyelonephritis is associated with marked suppression of CMI and that the latter can be used as an additional criterion for establishing the level of infection. Patients with UTI did not generally appear to have any primary defect of CMI but when suppression of CMI was present, it seemed secondary to an ongoing infect