The cellular mechanisms responsible for contractile dysfunction associated with atrial fibrillation (AF) are still poorly understood. Atrial fibrillation is often preceded by atrial dilatation. This study aimed to explain contractile alterations associated with AF and their relation to atrial dilatation, by studying the relationships between atrial dimensions, contractile protein composition, force production and Ca2+-sensitivity. Force development was determined in mechanically isolated single skinned cardiomyocytes from right atrial appendages from patients with sinus rhythm without (SR;n = 9), or with atrial dilation (SR + AD;n = 11) or atrial fibrillation (AF;n = 16). Echocardiography showed that, compared to the SR group, mean right atrial dimensions were increased by 18% and 35% in the SR + AD and AF group, respectively (P < 0.05). Protein composition was determined by 1- and 2-dimensional gel electrophoresis. Compared to the SR group, the AF group exhibited: a reduction in the kinetics of force redevelopment (Ktr) in isolated atrial cardiomyocytes, enhanced protein expression of the slow myosin heavy chain isoform (β-MHC), an increase in troponin T (TnT) phosphorylation and a marked increase (70%) of the cytoskeletal protein desmin. Significant correlations were observed between the right atrial major axis (RAmajor) and β-MHC expression as well as the desmin/actin ratio. Our findings indicate that dilatation may influence cardiomyocyte stability through altered desmin expression, but that it does not predispose to the alterations in contractile function observed in AF.