Prospective uterine fluid sampling in 140 patients was performed using a Wallace catheter. The levels of human decidua-associated protein (hDP)-200 in the uterine fluid and in the serum, and the total protein concentration in the uterine fluid were determined. Uterine fluid volumes were scored during the hysteroscopy and no correlation was found with the phase of the menstrual cycle. The total protein concentration in the uterine fluid did not vary significantly during the menstrual cycle. The hDP-200 levels in uterine fluid were significantly higher than those in the serum, although considerable individual variation in the level of uterine fluid hDP-200 was found.