We have measured the effects of Org 10172 (a mixture of naturally occurring glycosaminoglycans derived from hog intestinal mucosa) on blood loss after transurethral prostatectomy (TURP), using doses which are likely to prevent postoperative venous thrombosis (VT). 48 patients entered a double-blind randomised pilot study: 18 were given subcutaneous (sc) injections of a placebo and 30 received sc Org 10172 (750 anti-Xa units/day, 500 units twice daily (bid), or 750 units bid, starting just before TURP and continued until discharge; 10 patients per group). No Org 10172 regimen increased peroperative blood loss but all caused a similar trend towards increased urinary bleeding after surgery. Since there was no apparent dose effect gradient, it was decided to pool the data from all three dosing blocks: this analysis showed that Org 10172 increased geometric mean blood loss during the first 2 days after surgery from 10.4 gm hemoglobin (Hgb; range = 3.2-71) to 20.5 gm Hgb (range = 1.9-147) (p = .005), an effect which retained its significance after allowing for two other major determinants of postoperative bleeding, the weight of prostate resected and the length of surgery, and also when pooling was restricted to the twice daily Org 10172 injection groups and their corresponding controls. Bleeding was not severe, but our results indicate a need for caution when considering the use of Org 10172 in this setting.