Sequential blood isolates from eight patients with 10 episodes of recurrent Staphylococcus aureus bacteremia were typed by restriction endonuclease analysis of plasmid DNA (REAP DNA fingerprinting) and immunoblotting. There were six early recurrences (within 2 months of stopping antimicrobial therapy) and four late recurrences. All early recurrences isolates were identical to initial isolates. These recurrences were defined as possible relapses. Three of four late recurrence isolates were different from the preceding isolates recovered from four patients. This was considered indicative of new infections. There was complete concordance between REAP DNA fingerprinting and immunoblot typing results. However, four isolates lacked plasmid DNA and could be typed only by immunoblotting. All initial isolates from different patients were different types by immunoblotting and by REAP DNA fingerprinting (except for those lacking plasmid DNA). The bacterial traits detected by these methods appear to be stable in vivo for up to 3 months. Relapsing infections were associated with the presence of intravascular foreign bodies and vancomycin therapy of the preceding episodes.