Atrial fibrillation (AF) is the most common sustained arrhythmia. Because heat shock proteins (Hsp) can protect cells from stress, we compared the levels of Hsp60, Hsp72, Hsc73, and Hsp27 in atrial myocardium from 17 patients with AF (8 paroxysmal and 9 persistent) and 7 controls in sinus rhythm (SR). Hsp60, Hsp72, and Hsc73 levels were not significantly different among the 3 groups. Hsp27 expression was slightly higher in paroxysmal AF than in SR and in persistent AF, and a borderline significant difference (P = 0.064) was seen between the paroxysmal and persistent AF subgroups. Hsp60 levels in the moderate, severe, and profound myolysis groups were significantly lower than the light myolysis group, but no differences were found in other Hsps. In summary, the data indicate that expression of Hsp27 and Hsc73 may be associated with different stages of AF and that Hsp60 also may be associated with the degree of atrial myolysis.