Abstract: This disclosure relates to the determination of blood loss during endoscopic operations, particularly, during transurethral resectomy. It is known in such operations to employ continuous irrigation using sterilized water which is fed into the body cavity through the endoscope and fed out again mixed with the lost blood. According to the disclosure a method is provided wherein the specific blood content is found from one of the characteristics of the outgoing water which alters with the blood content: to determine the blood lost this specific blood content is compared with the rate of flow of the monitored outgoing flushing water.