Objective To compare the effectiveness of lipid lowering drugs in lowering serum cholesterol concentrations. Design Cross sectional study. Setting 17 practices within 17 primary care groups in Trent region, United Kingdom. Participants Patients aged 35 years or over taking lipid lowering drugs and with at least two serum cholesterol concentrations recorded on computer. Main outcome measures Proportion of patients achieving serum cholesterol concentration of <5 mmol/l and mean percentage reduction in serum cholesterol concentration. Results 1353 of 2469 (54.8%) patients receiving lipid lowering treatment had a last recorded serum cholesterol concentration of <5 mmol/l. Significantly more patients taking statins achieved the target value for serum cholesterol (5 mmol/l) than those taking fibrates (1307 (57%) v 46 (26%); P < 0.0001). Atorvastatin and simvastatin were the most effective drugs in achieving the target. Lipid lowering drugs differed significantly for pretreatment serum cholesterol concentration, most recent cholesterol concentration, and the associated percentage reduction. Atorvastatin and simvastatin achieved the greatest percentage reduction in serum cholesterol concentrations (30.1%, 95% confidence interval 28.8% to 31.4% and 28.0%, 26.7% to 29.3%, respectively). Although the mean serum cholesterol concentrations in this unselected population tended to be higher than those in clinical trials, the percentage reduction was consistent with the trials. Conclusion The ability of individual statins to lower serum cholesterol concentration varied, with atorvastatin and simvastatin being the most effective. The percentage reductions agreed with those of randomised controlled trials indicating likely benefits in unselected patients in primary care. As the initial serum cholesterol concentrations were higher than those in randomised controlled trials, target serum cholesterol values of <5 mmol/l may be unrealistic even for patients on the most efficacious drugs. Also, the higher initial concentrations could mean that the absolute reduction in cardiovascular risk in primary care patients is greater than thought. Introduction Statins in patients with coronary heart disease help reduce further cardiovascular events and improve survival. We know relatively little about the comparative effectiveness of different lipid lowering drugs because studies that make direct comparisons of the drugs are uncommon. We have only been able to find one study comparing the effectiveness of statins. This randomised controlled trial compared five statins, at different doses, over an eight week period in 534 patients. It was published by the manufacturer whose statin, atorvastatin, was most effective. Therefore we compared the effectiveness of individual lipid lowering drugs in lowering serum cholesterol concentration outside the setting of a clinical trial.