Substudy II examined whether propranolol had a favorable impact on STD status from baseline to repeat examination. The design of this substudy was prospective. One hundred fifty patients treated with propranolol and 150 untreated controls were randomly selected from among the 1,000 patients who had repeat 24-hour Holter monitoring at the six week follow-up visit. Patients were followed from baseline to repeat monitoring at six weeks with the STD durations at baseline and repeat monitoring being followed for each patient.