The effects of noscomial infection on duration of hospital stay and outcome of hospitalization were measured by matching two control patients to each of 85 patients found to have qctive nosocomial infection during a prevalence survey at Boston City Hospital in 1973. The control patients were selected from all patients discharged from this hospital during the same time period; they were matched by exact primary discharge diagnosis, similar operative procedure, and age. Patients with a single infection remained in hospital on average 13.0 days longer than their matched controls, and those with two such infections stayed on average 35.4 days longer. This effect of extra stay associated with nosocomial infection was consistent when data were stratified by primary discharge diagnosis, hospital service, site of infection, or outcome of hospitalization. The outcome of hospitalization for these infected patients was slightly, but not significantly, worse than for their matched controls.