The cardiac myosin phenotype, an important determinant of myocardial contractility, is modified by chronic increases in hemodynamic load. To quantify the proportion of atrial alpha-myosin heavy chain in various types of left atrial overload and to assess the possible relation between this proportion and atrial size, 34 patients were studied, 4 with Wolff-Parkinson-White syndrome, 29 with various types of mitral valve dysfunction and 1 with an atrial septal defect. Four normal autopsy hearts were also studied. The proportion of alpha-myosin heavy chain among total (alpha plus beta) myosin heavy chains was determined in each atrial sample, using an enzyme-linked immunosorbent assay. The size of the left atrium was assessed by one- and two-dimensional echocardiography. Alpha-myosin heavy chain was the main isoform present in the normal atria (85.5 ± 9% of total myosin heavy chains). Patients with pure tight mitral stenosis (n = 9), mitral stenosis plus mild regurgitation (n = 8) and severe mitral regurgitation (n = 8), who had a higher indexed left atrial transverse diameter than those with Wolff-Parkinson-White syndrome (33 ± 6, 39 ± 10 and 46 ± 5 versus 19.5 ± 2 mm/m2, p < 0.01, p < 0.001 and p < 0.001, respectively), also demonstrated a much smaller percent of alpha-myosin heavy chain content (28 ± 20, 23.5 ± 13 and 12 ± 10 versus 58 ± 18%, p < 0.01, p < 0.01 and p < 0.001, respectively). Significant negative linear correlations were observed between the percent alpha-myosin heavy chain content and either the indexed left atrial transverse diameter (r2 = 0.54, r = 0.73, p < 0.001) or the indexed left atrial area (r2 = 0.43, r = 0.66, p < 0.001). These data demonstrate a profound transformation of myosin phenotype in the chronically overloaded human left atrium and suggest that isomyosin transitions are related to the severity or duration, or both, of the atrial overload, whatever its type. © 1987, American College of Cardiology Foundation. All rights reserved.