The determination of the metabolites of histamine, 1-methylhistamine (MHIS) and 1-methylimidazoleacetic acid (MIIA), in human urine is a useful tool for the diagnosis of mastocytosis. MHIS was extracted under basic conditions with chloroform and derivatized with trifluoroacetic acid anhydride, MIIA was derivatized with pentafluorobenzylbromide prior to extraction under basic conditions and the derivative was extracted with chloroform. The samples were assayed by gas chromatography-mass spectrometry. Normal concentrations of MHIS (2.01 micromol l(-1)) and MIIA (21.3 micromol l(-1)) in healthy volunteers' urine samples are clearly detectable with coefficients of variation of 7.0 and 8.9%. Pathological concentrations of 10.1 and 113 micromol l(-1) for MHIS and MIIA, respectively, are quantifiable with coefficients of variation of 6.6 and 4.8%.