Four patients with osteoarthritis of the hip, who developed septic arthritis of the affected joint, are reported. The septic arthritis developed insidiously and was diagnosed with difficulty. One patient died, two required subsequent joint excision, and one arthrodesis despite antibiotics. The presence of a recognised predisposing factor to septic arthritis, such as rheumatoid arthritis or a surgically replaced joint, can provide a diagnostic pointer. These cases suggest that osteoarthritis, similarly, is a predisposing factor. It is concluded that joint sepsis should be considered if a patient with osteoarthritis develops new symptoms from a single joint with associated systemic features.