Most of the data for this paper were taken from a 30-year follow-up of septic arthritis and a 25 year series of osteomyelitis seen in Dallas, Texas. A specific bacterial aetiology was determined in about 70% of patients with septic arthritis, utilizing culture of multiple fluids. The aetiological agent was influenced by the age of the patient. In the newborn Staph. aureus, group B streptococci and gram-negative organisms are found most commonly. In the older infant H. influenzae becomes a prominent pathogen, and in those over 2 years of age staphylococci, streptococci, H. influenzae and N. gonorrhoea are the predominant organisms. Ninety-three per cent of arthritis was monoarticular. A slightly higher percentage (78%) of children with osteomyelitis had a specific bacterial aetiology determined. Staph. aureus was the most common pathogen found at all ages, averaging 53% of all cases and in all age groups, and followed in frequency by various types of streptococci. Diagnosis was delayed in osteomyelitis compared to septic arthritis. In the Dallas patients a single bone was involved in 316 compared to 24 with polyosteal disease. In both infections the initial antibiotic chosen is determined by the gram strain of material obtained from joint aspiration, pus, or other secretions. If no specific bacterial aetiology is found, treatment is begun for the most likely organism considering the age of the patient and the clinical situation. Recently oral therapy has been used extensively in specific instances after an initial period of parenteral therapy. A limited number of follow-up studies have shown that the age of the patient, the bone and/or joint involved, and the organism responsible all influence the long-term results in both septic arthritis and osteomyelitis. The poorest long-term prognosis is in the neonate, especially where the hip joint is involved either alone or with a concomitant osteomyelitis.