A 15-month-old boy was admitted with fever and a swollen knee. His mother had been treated for brucellosis 11 months ago. At that time he had been asymptomatic and had both negative blood culture and serum agglutination tests and breastfeeding had been stopped. The infant had been healthy since then. On admission, blood and joint fluid were obtained for culture and he was commenced on cefuroxim. Gram-negative coccobacillary organisms were seen in the joint fluid. Both cultures remained sterile. He had a positive serum agglutination test with a titer of 1/640. Cefuroxim was then stopped and the child was commenced on a specific course of treatment: gentamicin, trimethoprim-sulfamethoxazole and rifampicin. The infant recovered with this treatment. We conclude that this was brucellar arthritis of the knee and was probably acquired by breastmilk after an exceptionally long incubation period.