The menstrual cycle is characterised by variations in the absolute and relative concentrations of the hormones of the hypothalamic pituitary ovarian axis, which in turn affect cell function and cytokine and heat shock protein production. Menstruation involves the shedding of the secretory endometrium, which is part of the mucosal associated lymphoid tissue and hence is rich in immunologically competent cells such as CD8 T cells and macrophages. The case is reported here of a patient presenting with a recurrent but transient symmetrical inflammatory polyarthritis which only occurred at menstruation with no residual damage. The disease was suppressed by danazol. Endometrial degradation products are suggested as the trigger of this 'menstrual arthritis'.