Lesion size during radiofrequency catheter ablation in patients with paroxysmal supraventricular tachycardia (PSVT) is thought to be related to multiple factors, including contact pressure at the catheter-endocardial interface. Therefore a predictor of contact pressure at a potential target site for ablation might be useful. In this study 25 patients underwent duplicate 2 W applications of radiofrequency energy with the catheter in poor and firm contact with the right ventricular endocardium after successful ablation treatment for PSVT. The mean age of the patients was 44 +/- 15 years. Fifteen patients underwent slow pathway ablation for atrioventricular nodal reentrant tachycardia, and 10 patients underwent ablation for an accessory pathway. The mean impedance for low-energy applications in firm contact (139 +/- 24 ohms) was 22% +/- 13% greater (p 0.0001) than in poor contact with the right ventricle (113 +/- 16 ohms). The maximum impedance was 27% greater when the catheter was in firm (147 +/- 28 ohms) rather than poor contact (116 +/- 16 ohms), with the endocardium (p 0.0001). These results suggest that higher impedance measurements may be obtained with low-energy applications of 2 W when the ablation catheter is in firm contact with the endocardium.