In a series of 90 patients with chronic pain syndromes of psychogenic and organic etiology, the concentrations of fraction I endorphins in cerebrospinal fluid were investigated. A significant circannual variation in the concentrations of endorphins was found, with the highest concentrations in January-February and the lowest concentrations in July-August. There was no corresponding seasonal variation with regard to age, sex, bodylength, possible etiology of the pain syndrome self-rated pain levels, or experimental pain measures. Circannual difference in the intensity of symptoms in chronic pain syndromes and in affective disorders have been described in the literature. The present results suggest an association between these observations, giving further support for functional importance of endorphins in chronic pain.