Between June 15 and August 7, 1982, six patients in a medical ICU developed primary nosocomial bloodstream infections. Nine different organisms were isolated during this epidemic. In three patients, contamination of intravascular lines was documented; and in the other three, the relationship was strongly suggested. The infected patients were compared to three groups of randomly selected controls: group A from the epidemic period; group B from the pre-epidemic period; and group C from both periods and matched for underlying disease. The infected patients had indwelling arterial and pulmonary artery catheters in place almost twice as long (6.3 +/- 2.8 days and 6.8 +/- 3.9 days, respectively) as most controls. Our evidence suggests that the guideline for changing catheters may have been ignored when the resident house staff changed. This is the first outbreak related to vascular catheters in which several different organisms were involved; it represents a new type of epidemic related to a common technique problem rather than a common source reservoir.