Leukotrienes (LTs) are active lipid mediators derived in the 5-lipoxygenase pathway. LTC4, the primary cysteinyl LT, is cleaved by γ-glutamyl transpeptidase (GGT), resulting in LTD4. We studied the synthesis and metabolism of LTs in three patients with GGT deficiency. LTs were analyzed in urine, plasma, and monocytes after HPLC separation by enzyme immunoassays, radioactivity detection, and electrospray tandem mass spectrometry. Analysis of LTs in urine revealed increased concentrations of LTC4 (12.8–17.9 nmol/mol creatinine; controls,