Three cases of neonatal septic osteo-arthritis due to umbilical artery catheterisation are described. The clinical features were characteristic. A week after removal of the catheter a septic osteo-arthritis developed. The etiological agent in all three cases was Staphylococcus aureus, resistant to penicillin. The disease is probably caused by haematogenous spread of septic clots from thrombus formation on the catheter. Symptoms and signs were few and vague. It is suggested that septic osteo-arthritis due to umbilical catheterisation is more frequent than hitherto believed, and low-grade forms might remain undiagnosed.