Eighteen patients (mean age, 66 years) who were receiving oral potassium supplements while being treated with diuretics were enrolled in this study. The patients were randomly assigned to receive Slow-K, a wax-matrix tablet, or K-Dur, a microencapsulated tablet, for two weeks and then the other drug for an additional two weeks; the daily dose was two to four tablets, equivalent to 20 to 40 mEq of potassium. Patients were asked to complete a survey about their compliance and preference ratings of both products. Of the 17 patients who completed the study, 14 stated that they preferred Slow-K to K-Dur and one preferred K-Dur to Slow-K; 14 patients made positive comments about Slow-K and seven made positive comments about K-Dur. The most common positive comment about Slow-K was its small size and ease in swallowing. Adverse reactions (mostly gastrointestinal) were reported by five patients during treatment with Slow-K and by six during treatment with K-Dur; three patients treated with K-Dur and none treated with Slow-K discontinued treatment because of adverse reactions.