Fabry disease is a complex, multisystemic and clinically heterogeneous disease, with elevated excretion of globotriaosylceramide (Gb(3)) and globotriaosylsphingosine (lyso-Gb(3)) accumulating in biological fluids caused by deficiency of the enzyme, lysosomal α-galactosidase A. Our aims were to propose a tandem mass spectrometry fragmentation mechanism for lyso-Gb(3), to develop and validate a simple, and robust methodology for the measurement of plasma lyso-Gb(3) using LC-MS/MS in large Fabry cohorts and in controls. Response to treatment was also evaluated.A solid-phase extraction procedure was used to process plasma samples. The 1-β-D-glucosylsphingosine (GSG) internal standard was chosen for its commercial availability. A liquid chromatography method was devised to allow the co-elution of the GSG internal standard with lyso-Gb(3), thus compensating for system variability and reducing the matrix effect. A multiple reaction monitoring method was developed, working in positive electrospray ionization.The validation of the method provided good accuracy and precision: intraday and interday biases of less than 8% and 5%, respectively, and intraday and interday CVs of <12% and 7%, respectively. Limit of detection was 0.7 nmol/l and limit of quantification was 2.5 nmol/l. Plasma samples were stable for up to 6h at room temperature, 48 h at 4 °C, and 20 weeks at -20 °C. Regarding untreated Fabry patients, the mean lyso-Gb(3) concentrations were 170 nmol/l for males and 9.7 nmol/l for females, and for treated patients, 40.2 nmol/l for males and 7.5 nmol/l for females.A robust LC-MS/MS methodology is presented for plasma lyso-Gb(3) quantification.