Previously we established that a cocoa-enriched diet in young rats reduces the specific antibody production and the Th lymphocyte proportion in lymphoid tissues. The aim of the present study was to ascertain the modulatory ability of a cocoa flavonoid enriched diet on collagen-induced arthritis (CIA), which is mediated by anti collagen autoantibody response and Th lymphocyte activation. Female LOU rats were fed with a cocoa enriched diet, beginning two weeks before CIA induction. The hind paw swelling and serum cytokine and anti collagen antibody concentrations were determined. Anti collagen antibody-secreting cell counts and lymphocyte subset proportions were established in inguinal lymph nodes. Reactive oxygen species (ROS), NO and TNFα by peritoneal macrophages were determined. Although arthritis cocoa-fed rats showed a similar hind-paw swelling time course as the arthritis animals fed a standard diet, the cocoa intake was able to decrease specific IgG2a, IgG2b, and IgG2c titres. Moreover, cocoa intake in CIA rats reduced ROS production, TNFα and NO release from peritoneal macrophages, and decreased the Th/Tc ratio in inguinal lymph nodes. In conclusion, a cocoa flavonoid-enriched diet in LOU rats with CIA produced no effect on hind-paw swelling but was able to modulate the specific antibody response and also the Th lymphocyte proportions, as well as the synthesis of pro-inflammatory mediators from peritoneal macrophages. Therefore, a cocoa-enriched diet could be a good adjuvant therapy in disorders with oxidative stress or autoimmune pathogenesis.