We compared the efficacy of diltiazem and propranolol in a randomized, prospective, double-blind, crossover study in six patients with untreated thyrotoxicosis (mean age 31 yrs). The patients received either diltiazem 60 mg orally four times a day or propranolol 40 mg orally four times a day, each for 1 week, separated by a 3-day drug-free period. Blood pressure, heart rate, thyroid hormone levels (free T4, T3), electrocardiogram, two-dimensional and M-mode echocardiograms, and Doppler studies were performed. In addition, 8 clinical signs and 18 symptoms of thyrotoxicosis were graded. All subjects felt better with drug therapy, with three preferring diltiazem to propranolol. No significant difference in clinical response or in hemodynamic effects was noted between the agents. These data suggest that diltiazem may serve as an alternative therapy for beta blockers in controlling thyrotoxic symptoms in patients in whom beta blockade may be contraindicated.