We report a retrospective study of 94 infants, ages < 4 months, who underwent investigation for possible osteomyelitis during a 9-year period. Of the 30 babies with proven osteomyelitis (radiographic changes or positive bone cultures or positive blood cultures plus a compatible clinical picture), 17 were preterm artificially ventilated babies and 4 were full term infants receiving intensive care. An etiologic organism was isolated from 28: methicillin-susceptible Staphylococcus aureus, 16; methicillin-resistant S. aureus (MRSA), 7; Escherichia coli, 3; and group B Streptococcus, 2. MRSA occurred exclusively in the preterm group. Osteomyelitis was multifocal in 40% and associated with septic arthritis in 47%. The long bones were frequently affected (80%) whereas the flat bones were often sites of clinically silent disease. Twenty-five (83.3%) of the 30 babies with proven osteomyelitis had focal clinical signs or evidence of disseminated staphylococcal disease. Only 10 were febrile. Four of 27 babies investigated because of positive blood cultures for S. aureus but no focal signs had osteomyelitis, as did only 1 of 27 babies with suspected sepsis but no focal signs. The sensitivity of 99mTc bone scanning was 84%, specificity 89%, positive predictive value 79% and negative predictive value 92%. The addition of gallium scanning (in 39 of the 94 infants) improved the respective figures to 90, 97, 93 and 95% and was useful in interpreting equivocal bone scans.