An outbreak of Nocardia asteroides infection occurred in the nephro-urological intensive care unit, St. Peter's Group of Hospitals, London in 1979. In is suggested that the outbreak has been due to patient-to-patient transmission. Five months before the outbreak. N. asteroides was first isolated from nephrostomy urine from an asymptomatic patient who had undergone several urological operations. Subsequently six of the seven patients admitted for renal transplant over a three month period developed proven infections. The first transplant patient had an abdominal abscess and the other five had only pulmonary disease with obvious radiological lesions. Bacteriological diagnosis was made in three patients by examination of sputum and in the other three from bronchoscopic specimen, pleural fluid and pus from an abscess respectively. Five of the six renal transplant patients were treated with amoxycillin and erythromycin. Clinical and radiological improvement was observed and no patient died as a result of the infection. In the intensive care unit air and dust samplings were positive for N. Asteroides. The unit was closed and fumigated with formaldehyde and when the air sampling continued to be negative the unit was then re-opened.