Background A histopathological assessment of left atrial appendages (LAA) resected during surgical treatment for atrial fibrillation (AF) was made, with the aim of improving the prediction of postoperative AF recurrence. Methods and Results This clinicopathological study involved 57 surgical cases of valvular AF and 34 age- and sex-matched control autopsy cases with a history of sinus rhythm. LAA from the cases with valvular AF showed greater hypertrophy of cardiomyocytes (p<0.0001), greater nuclear enlargement (p<0.0001), more bizarre nuclei (BN; p<0.0001), and more intercellular fibrosis (ICF; p<0.001). Partial disarray of cardiomyocytes and fatty infiltration were recognized in both the AF and control groups. Thirty-seven cases had maintained sinus rhythm after surgery from 7 months to 10 years. AF recurred within a month of surgery in 17 and after a month in 3; there was no significant difference in histopathological features between them. These 20 cases had more cellular hypertrophy (p<0.025), nuclear enlargement (p<0.025), BN (p<0.01), and ICF (p<0.025) than those who maintained sinus rhythm after surgery. Conclusions The histopathological findings for LAA reflected the underlying valvular diseases; however, the most reliable predictors of postoperative AF recurrence were hypertrophy of cardiomyocytes, bizarre shaped nuclei, and extensive ICF. (Circ J 2007; 71: 70 - 78)