One hundred and eighty-one isolations of nonpigmented Serratia marcescens from 104 patients were made during a one-year period, most of them from urines and sputa. Patients with positive cultures had either a preexisting local disease, a history of instrumentation (catheterization, treatment with an intermittent positive pressure breathing device) or operation, or both. Other contributing factors were impaired general resistance and preceding antibiotic treatment. An evaluation of the role of Serratia in the course of each case proved difficult. However, the virulence of our strains seemed to be low. Few symptoms could be attributed exclusively to Serratia infections. Emergence after antibiotic treatment, resistance to many antibiotics, prevalence in hospital patients, and typical ward epidemics suggest that Serratia is a predominantly hospital-acquired organism. It may constitute a potential danger to debilitated patients, and its finding should not be dismissed as meaningless.