A prospective study was carried out in a polyvalent medical intensive care unit to determine the role played by diagnostic and therapeutic invasive techniques in the development of four nosocomial infections. Over a 6-month period, 206 patients admitted for more than 48 hours were studied; all had undergone one or several invasive techniques: 76 patients (37 per cent) developed a total of 117 nosocomial infections including urinary tract infection (41.8 per cent), purulent bronchitis (37.5 per cent) septicaemia (11.6 per cent) and pneumonia (7.6 per cent). The risk factors were: duration of urinary or vascular catheterization, tracheal intubation, tracheotomy and artificial ventilation. Gram-negative bacilli (59 per cent) predominated over Gram-positive cocci (39.6 per cent).