A 49 year old man is described with a polyarticular arthritis. Synovial fluid aspirated from the knee joint showed monosodium urate monohydrate and calcium pyrophosphate dihydrate by polarised light microscopy. Additionally, diphosphonate binding and scanning electron microscopy with energy dispersive analysis showed that basic calcium phosphate crystals were also present. This appears to be the first report of three crystals occurring simultaneously in a single joint.