SummaryTwelve patients with primary hyperlipoproteinemia type IIaaccording to Fredrickson were treated after a six week placebo period with a new cholesterol-lowering resin. Colestipol (U-26.597 A, Upjohn). The drug was given orally, five g three times daily. Ten patients completed the trial and were treated with Colestipol during one year. From the third month of treatment a significant decrease of their cholesterolemia was observed (P<0.05). This cholesterollowering effect persisted after twelve months with a mean decrease at the end of the trial of 16.6%. Serum triglycerides, glycemia and parameters of the calcium and phosphate metabolism were not influenced by the resin. A decrease in uricemia was noticed. The two patients with an abnormal oral Glucose Tolerance Test responded less or not at all. One patient interrupted the treatment due to gastro-intestinal complaints. The other patients did not show any untoward reactions except for episodes of moderate constipation. There were no signs of toxicity.