The 127 diet-resistant primary hyperlipidemic patients received 100 mg of ciprofibrate daily for 12 weeks. In the 63 patients with type IIa hyperlipidemia and 41 patients with type IIb hyperlipidemia, serum levels of total cholesterol, very-low-density lipoprotein cholesterol, low-density lipoprotein cholesterol, triglycerides, very-low-density lipoprotein triglycerides, and apolipoprotein (apo) B decreased significantly and levels of high-density lipoprotein cholesterol and apo A-I increased significantly. Similar changes occurred in the 23 type IV patients, except that high-density lipoprotein cholesterol levels increased significantly and apo B levels did not change. No clinically significant side effects or drug-related abnormal laboratory test results were noted. It is concluded that ciprofibrate is a safe and potent hypolipidemic agent.