The records of 28 patients undergoing splenectomy for myeloid metaplasia were reviewed. Portal and mesenteric venous thrombosis was proved or suspected in five patients. The time interval between splenectomy and portal and mesenteric venous thrombosis varied from six days to three years. The incidental postmortem finding of portal vein thrombosis in one patient suggests that the incidence may be greater than clinically appreciated and may, at times, be asymptomatic. Postsplenectomy thrombocytosis was not found to be a significant risk factor, occurring in only one of the five patients in whom portal vein thrombosis developed. Recommendations for prevention include ligation of the splenic vein close to its junction with the inferior mesenteric vein at the time of splenectomy. In addition, prophylactic anticoagulation therapy or the use of antiplatelet drug therapy postoperatively, or both, should be considered.