From 1987 to 1993 the Swiss Group for Clin. Cancer Research (SAKK) performed a randomized phase III adjuvant trial in patients with colorectal adenocarcinoma with the objective of comparing intraportal vs. i.v. perioperative chemotherapy. Patients <75 yr of age had a curative surgical resection of their cancer and were then randomized into 3 group: (1) adjuvant perioperative portal liver infusion with fluorouracil, mitomycin and heparin, (2) systemic i.v. infusion with the same regimen, and (3). no adjuvant treatment. The hematol. toxicity was evaluated by Hb determination and leukocyte and thrombocyte counting before and during the 1st 10 days after surgery. After surgery, Hb showed a median decrease of 22% in the control group. This decrease was aggravated by 3% by the chemotherapy. Leukocytes showed a median decrease of 7% after surgery in the control group. Perioperative chemotherapy caused a higher median drop: 23% when given into the liver through the portal vein and 34% when given systemically through a subclavian catheter. Thrombocytes had a median postsurgical decrease of 25% in the control group. Chemotherapy was not associated with a significant addnl. drop. Thus, adjuvant perioperative chemotherapy with fluorouracil, mitomycin and heparin was associated with a significant mild drop in Hb and leukocytes during the 1st 10 postoperative days.