BACKGROUND The small heat shock protein 27 (hsp27) increases in expression with ischemia/reperfusion (I/R) insult in the heart. One feature of the small hsps is their ability to oligomerize and form intracellular aggregates. Oligomerization pattern is governed by the phosphorylation state of the protein that may influence their ability to protect against cellular stresses. RESULTS We generated transgenic (tg) mice that overexpress a wild-type human hsp27 (hsp27tg) protein or a mutant hsp27 protein (mut-hsp27tg), in which serine residues (aa15, aa78, and aa82) were replaced by alanine residues, rendering them incapable of phosphorylation. Using a Langendorff perfusion model and an intraventricular balloon, we subjected hearts to 20 minutes of ischemia followed by 1 hour of reperfusion. During reperfusion, negative and positive pressure derivatives as well as developed pressures were significantly higher in both hsp27tg and mut-hsp27tg compared with control (P<0.01) mice, with no significant difference between hsp27tg and mut-hsp27tg. Creatine kinase release during reperfusion was higher in control compared with both hsp27tg and mut-hsp27tg (P<0.05). Malondialdehyde content as well as protein oxidation products were lower in mut-hsp27tg compared with control (P<0.05). hsp27tg hearts possessed oligomers that ranged in size from small to large, whereas mut-hsp27tg hearts contained no small oligomers. CONCLUSIONS These results indicate that in a tg mouse model, overexpression of either wild-type hsp27 or a nonphosphorylatable hsp27 mutant was equally capable of protecting the heart from I/R injury. Furthermore, the phosphorylation status of hsp27 may influence its ability to decrease oxidative stress.