Awareness of the virulence of coagulase-negative Staphylococci, previously regarded as saprophytes with minimal pathogenicity, has steadily increased. Eighty-seven individual patients diagnosed with acute osteomyelitis, as confirmed by microbiologic and pathologic analysis, were included in this study. Of these patients, 82% (71/87) were known to have diabetes mellitus. The prevalence of coagulase negative Staphylococcus was 40% (35/87) in deep bone cultures, 63% (22/35) of which were methicillin resistant. When the coagulase negative Staphylococcus group was assessed for prior long-term (> 2 week) oral antibiotic treatment with ciprofloxacin, it was found that 54% (12/22) of the methicillin-resistant coagulase-negative Staphylococcal infected patients had received such treatment, compared with 15% (2/13) of patients with methicillin-sensitive coagulase-negative Staphylococcal osteomyelitis (p < 0.034). When the group was analyzed for prior long-term antibiotic treatment with amoxicillin/clavulanate, 23% (5/22) of the methicillin-resistant patients had received oral amoxicillin/clavulanate, compared with 23% (3/13) of patients with methicillin-sensitive coagulase-negative Staphylococcal osteomyelitis (p > 0.05). Prevalence of polymicrobial infections, which constituted 29% (25/87) of all individual patients, was also analyzed. Of those patients with coagulase-negative isolates, 29% (10/35) were polymicrobial (p > 0.05). The results from this study suggest that infections of bone caused by coagulase-negative Staphylococci are associated with a high prevalence of methicillin resistance. This study also raises the question of whether injudicious prolonged use of ciprofloxacin may, in fact, promote proliferation of resistant organism strains.