OBJECTIVE To define the source of invasive coagulase-negative staphylococci (CNS) and the epidemiology of strain variation in immunocompromised patients. METHODS Weekly microbial surveillance cultures were obtained from the nares, throat, skin, rectum, and urine. Plasmid pattern analysis was performed on all coagulase-negative staphylococci isolated from blood cultures and on selected strains from the surveillance sites. METHODS A 902-bed, university-owned, tertiary-care referral hospital. METHODS Forty-four patients on the bone marrow transplant or hematologic malignancy services. RESULTS Plasmid pattern analysis was performed on 340 surveillance isolates (median = 7 per patient) and 201 bloodstream isolates (median = 3 per patient). Patients were colonized with numerous unique strains (median = 5 per patient) of coagulase-negative staphylococci. The 44 patients had 108 episodes of positive blood cultures, 20 of which were preceded by colonization with the same strain. Isolation of the matching strain from surveillance cultures preceded the positive blood culture by 1 to 8 days in 9 episodes and 18 to 389 days in 11 episodes. The matching strain was isolated from the skin in only 6 (30%) of those episodes and from mucosal sites in 70%. Of the 108 episodes of positive blood cultures, 21 were identified as nosocomial bloodstream infections. Four of the 21 nosocomial bloodstream infections were preceded by colonization with the same strain. In all 4 episodes, the infecting strain was cultured from the nares before the blood cultures were obtained. CONCLUSIONS Our results suggest that mucous membranes might be sources for strains of CNS causing bacteremia.