To evaluate whether the identification of the different types of relations between premature ventricular contractions (PVCs) and the preceding sinus cycle length is capable of predicting the effect of beta-blockers on the PVCs themselves, 55 patients (43 men, 12 women, mean age 52.6 +/- 15.6 years) with different cardiac diseases, and >30 PVCs/hr characterized by stability and the same relation at two Holter monitoring periods were studied. The relation was tachycardia enhanced (the shorter the preceding cycle length, the higher the incidence of PVCs) in 23 patients (group 1); indifferent (no correlation between the preceding cycle length and PVC incidence) in 21 (group 2); and bradycardia enhanced (the longer the preceding cycle length, the higher the incidence of PVCs) in 11 (group 3). A third Holter monitoring was performed 6 days after nadolol administration (80 mg/day) to evaluate its effect on the three types of PVCs. Incidence in all patients (-88;p<0.001). In group 2, it caused a reduction in the majority of patients (-60%;p<0.05) but an increase in five. In group 3, it caused a reduction in only half of the patients (-45%) and a 91% increase in the remainder. The difference in the effect of nadolol in the three groups was highly significant (X2=27.5;p<0.0001). The relation between the incidence of PVCs and the preceding cycle length is a useful means of identifying subsets of patients with PVCs who will benefit from beta-blockers.