Although I have a special interest in venous thrombosis, including upper extremity thrombosis, I have not previously considered the implications of catheter size vs vein size when ordering a PICC. My major concern has been to avoid the complications of fluid infiltration, infection, and thrombosis. I have favored brisk flow of fluids through the PICC and, therefore, have gravitated toward the largest lumen catheter that is practical for a particular patient’s vein size. My clinical focus has centered on the inner lumen diameter size of the catheter, rather than the overall internal plus external size of the catheter. Brisk flow through the internal lumen of the catheter makes it unlikely that the infusion pump alarm will sound a warning that the ordered amount of fluid cannot be infused. However, a large PICC is susceptible to the paradoxical effect of slow flow due to venous obstruction. Under these circumstances, flow slows presumably because the catheter tip is at least partially obstructed. Such a catheter must be causing at least some trauma to the vein wall. Thus, despite the best of intentions, conditions for a “perfect storm” can arise and predispose the patient to upper extremity venous thrombosis.