A 15- year-old male patient presented with mental retardation, mild motor impairment, and partial deafness. Biochemical investigations showed an abnormal urinary profile of leukotrienes. Concentration of leukotriene D4 (LTD4), which is usually not detectable, was highly increased, whereas LTE4,the major urinary metabolite in humans, was completely absent. These data suggest membrane-bound dipeptidase deficiency, a new defect in leukotriene biosynthesis on the step of LTE4 synthesis, as underlying defect. A 15-year-old male patient presented with mental retardation, mild motor impairment, and partial deafness. Biochemical investigations showed an abnormal urinary profile of leukotrienes. Concentration of leukotriene D4 (LTD4), which is usually not detectable, was highly increased LTE4, the major urinary metabolite in humans, was completely absent. These data suggest membrane-bound dipeptidase deficiency, a new defect in leukotriene biosynthesis on the step of LTE4 synthesis, as underlying defect.