Granulocyte-macrophage colony stimulating factor (GM-CSF) has been used to augment circulating hemopoietic stem cells in the peripheral blood and promote efficient peripheral stem cell (PSC) collection. Anecdotal reports indicate that GM-CSF may contribute to apheresis catheter dysfunction during PSC collection. We prospectively evaluated thrombotic occlusion rate of inferior vena cava (IVC) catheters following introduction of GM-CSF compared with a control group who underwent PSC collection prior to introduction of GM-CSF. Both groups were given aspirin (325 mg daily). The active treatment group was given GM-CSF (starting dose 250 micrograms/m2/day) as continuous infusion via the IVC catheter. The rate of thrombotically occluded catheters increased from 1/29 (3%) to 20/37 (54%) (p < 0.0001). Thrombi developing during GM-CSF therapy were unusual as 25/36 (69%) occlusions cleared with mechanical clot aspiration and did not require lytic agents.