In previous studies a tumor-size dependent decline of the circadian amplitude of serum melatonin was found in primary unoperated breast cancer patients, which was not due to changes of the hepatic metabolism of melatonin since its main peripheral metabolite, 6-sulphatoxymelatonin (aMT6s), showed similar serum levels. The aim of the current study was to verify these previous results by measurements of the nocturnal excretion of aMT6s in urine. The determination of aMT6s was carried out by radioimmunoassay. 17 primary unoperated breast cancer (BC) patients and 34 age-matched control patients with different types of benign gynecological diseases awaiting operation (breast diseases, n=13; ovarian diseases, n=12; and uterine diseases, n=9) were analysed. The median nocturnal urinary aMT6s excretion (22:00-6:00 hr) was significantly lower (-48%, P = 0.033) in BC patients than in controls. Controls showed a significant negative linear regression with age (r = -0.419, P = 0.014). According to multivariate linear regression analysis, BC revealed no age-dependency but a significant negative effect of increasing tumor-size on aMT6s-excretion (P = 0.036) was detected. These results confirm previous findings of a decreased pineal melatonin secretion in BC patients as well as an inverse relationship with tumor-size excluding a possible distortion due to age. The mechanisms involved are unknown but indicate that BC may lead to an impaired production of pineal melatonin. The clinical relevance of these findings from therapeutic and diagnostic point of view is discussed.