OBJECTIVE To analyze peroperative biopsies of RV myocardium in Tetralogy of Fallot by light microscopy, morphometry and electron microscopy in order to determine the degree of hypertrophy and degenerative changes and to correlate these changes with clinical and haemodynamic parameters. METHODS Right ventricular myocardium obtained peroperatively during surgical correction of Tetralogy of Fallot along with age-matched control samples were processed for routine light and electron microscopy using standard processing techniques. Mean cell diameter was analyzed using manual morphometric methods and ultrastructural study was carried out using a Philips transmission electron microscope. RESULTS The most consistent features of hypertrophy were the mitochondrial changes and increased nuclear convolutions. Majority of the patients had hypertrophy with mild to moderate degenerative changes. Severe degeneration was associated with irreversibility and was related to the severity and chronicity of the disease. There was a significant correlation of the morphological changes with clinical and haemodynamic parameters. CONCLUSIONS Peroperative histomorphometric and ultrastructural evaluation of the RV myocardium in Tetralogy of Fallot reflects the effect of haemodynamic stresses on the right ventricular muscle and correlates with clinical cardiac dysfunction. It may be a useful adjunct in determining the time for surgical intervention and in predicting clinical outcome.