This randomized, parallel-group, multicenter clinical trial compared a newly developed, once-daily, extended-release formulation of gemfibrozil (Lopid SR) and gemfibrozil twice daily (Lopid) in terms of lipid-regulating effects and toxicity. Patients were men and women with elevations of low-density lipoprotein cholesterol and low levels of high-density lipoprotein cholesterol. The trial consisted of a 1-week screening period, an 8-week diet baseline period (Step One Diet), and a 24-week double-blind treatment period (extended-release gemfibrozil 1,200 mg once daily vs gemfibrozil 600 mg twice daily). At the end of the trial, the 2 treatment groups showed comparable improvements in all primary lipid factors: mean percent changes in triglyceride, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol were -32, +10 and -10% for extended release (n = 325) and -36, +11 and -10% for twice daily (n = 330). The 90% confidence interval for the relative difference between the treatment means fell within the equivalence bounds of +/- 35% for all 3 factors, demonstrating equivalence of efficacy. Adverse events were reported at low rates and were similarly distributed in frequency and intensity between treatment groups; they were preponderantly mild or moderate, and gastrointestinal effects were the most frequent. The once-daily formulation of gemfibrozil may afford better control of dyslipidemia through improved compliance by patients who have this asymptomatic disease.