Delayed hypersensitivity skin testing was performed in 129 critically ill patients. Six intradermal antigens were used: tuberculin, candidin, varidase, epidermophytin, trichophytin and CCB (a polyvalent microbial vaccine from the Pasteur Institute). The response was judged as positive when one test or more were positive. Patients were devided in four groups: group A (40 cases): non-infected patients, a priori without immunodeficiency; group B (14 cases); suspected of immunodeficiency (cancers, hemopathies, collagen diseases receiving corticosteroids); group C (24 cases): decompensated chronic respiratory insufficienceis; group D (50 cases): overwhelming sepsis (septicaemias, septic acute respiratory distress syndromes, thoracic empyemas, purulent meningitis, peritonitis, mediastinitis). A significant diminution of delayed hypersensitivity was observed in groups B, C and D. No relation was found between delayed hypertensitivity and prognosis in groups C and D.