An important problem in investigating food allergic diseases is to adduce evidence that specific diseases of unknown cause may be attributable to food allergy. Polyarthritis is a good example of the difficulties involved in such studies because it is a very heterogeneous disorder and generalizations about aetiology are unlikely to prove correct. We have observed transient synovitis in both children and adults which is caused by food allergy. On the other hand we have not been able to show that food allergy demonstrably contributes to juvenile chronic arthritis or to rheumatoid arthritis in adults. The arthritis associated with inflammatory bowel disease undoubtedly responds to elemental diets and the mechanism of this remission warrants further investigation.