Atherosclerosis is one of the main causes of death in developed countries. Atheroma plaque formation is promoted by the interaction between the cells conforming the arterial wall, smooth muscle cells, and endothelial cells, together with lipoproteins and inflammatory cells (mainly macrophages and T-lymphocytes). These interactions can be mediated by proteins secreted from these cells, which therefore exert an important role in the atherosclerotic process. We recently described a novel strategy for the characterization of the human atherosclerotic plaque secretome, combining two-dimensional gel electrophoresis and mass spectrometry (MS). Among the identified proteins, two isoforms of heat shock protein 27 (HSP27), a protein recently described as a potential biomarker of atherosclerosis, were detected. However, the putative mechanisms in which HSP27 isoforms could be involved in the atherosclerotic process are unknown. Thus, the role that phosphorylated HSP27 could play in the atherosclerotic process is actually under study. The present work shows the strategies employed to characterize the phosphorylation in the HSP27 secreted by atheroma plaque samples. The application of liquid chromatography tandem mass spectrometry (MS/MS), as well as the combination of immobilized metal affinity chromatography methodology with matrix-assisted laser desorption/ionization MS/MS are described.