A new method for microquantification of phospholipid classes by nanoelectrospray mass spectrometry and stable isotope dilution is presented. The method covers the sum of phosphatidylcholine and sphingomyelin and in addition selectively quantifies phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. A phospholipid class is quantified together with its corresponding lyso-species due to the presence of a common head group. Phospholipids are extracted from tissue lysates, hydrolysed by hydrofluoric acid, and the liberated polar head groups choline, ethanolamine, serine, and inositol are quantified by nanoelectrospray mass spectrometry using deuterium-labeled analogs of the head groups as internal standards. The method is applied to tissue samples of a gastrointestinal tumor and of corresponding non-affected control tissue. In the tumor sample, the abovementioned phospholipids were found at roughly threefold elevated concentrations with a virtually unaltered relative abundance profile.