High dose methotrexate (MTX) is responsible of severe toxicity in patients in whom elimination from plasma is delayed. Factors responsible for MTX accumulation are partly known but some patients still experience toxicity despite adequate measures being taken. Our hypothesis is that renal tubular secretion may be impaired in these patients. This study aims at evaluating the performance of the UCP ratio (urinary ratio of coproporphyrins), a putative biomarker of tubular secretion, in predicting delayed MTX elimination.