To identify predictors of the success or failure of medical therapy in chronic recurrent sustained ventricular tachycardia, univariate and multivariate statistical techniques were used to retrospectively analyze data in 84 patients with this arrhythmia. By univariate analysis, four factors were associated with successful medical treatment: age less than 45 years, ejection fraction greater than 50%, hypokinesia as the only contraction abnormality and the absence of organic heart disease. Four other findings, the induction of ventricular tachycardia with a single ventricular extrastimulus, an HV interval greater than 60 ms, the presence of a left ventricular aneurysm and Q waves on a baseline electrocardiogram, correlated with medical failure. However, none of these variables alone accurately predicted treatment results in more than 75% of cases. By discriminant analysis, a function incorporating eight variables was constructed which correctly classified 81% of patients. Moreover, three-quarters of the patients could be classified into groups with a high or low probability of success where accuracy increased to 90%. When the discriminant function was tested prospectively in 31 similar patients, 25 (81%) fell into the groups with a high or low probability of success. In the latter group, of 20 patients predicted to fail medical therapy, 19 (95%) did fail a complete trial of medical therapy. The overall accuracy remained a high 92%. In clinical application this function would allow patients with a high probability of responding to medical therapy to be selected for serial electrophysiologic drug testing. In patients with a low probability of responding to medical therapy, serial studies could be avoided and alternate forms of therapy explored.