Aneurysms of the portal venous structures have rarely been reported. We present a case of a splenic vein aneurysm that appeared and regressed in parallel with splenic size during the course of a systemic infection in a patient with leukemia. The aneurysm was demonstrated with B-mode and color Doppler sonography and computed tomography. After appropriate therapy, the spleen diminished in size. This decrease was accompanied by regression of the aneurysm. Color Doppler sonography enables the noninvasive detection, diagnosis, and follow-up of splenic vein aneurysms. (C) 1998 John Wiley & Sons, Inc.