OBJECTIVE: To determine the number needed to treat (NNT) for one additional patient to achieve the European Atherosclerosis Society (EAS) target for LDL-C of <3 mmol/L at 12 weeks for rosuvastatin compared to atorvastatin, pravastatin or simvastatin. Patients included in the clinical trials comparing rosuvastatin with atorvastatin, pravastatin and simvastatin had an initial LDL-C =160 (4.1mmol/L) and <250mg/dL (6.5mmol/L). METHODS: The rosuvastatin trials were specifically designed to allow pooling of the data to provide summary estimates of efficacy. This design strategy minimised the potential bias caused when pooling results by variation in the event rates, differences in the outcomes considered, effects of secular trends on disease risk, and differences in clinical setting. The pooled efficacy data was used as the basis for the calculation of the NNTs. RESULTS: Rosuvastatin 10mg is more effective at lowering LDL-C to the EAS target than atorvastatin 10mg, pravastatin 20mg and simvastatin 20mg at 12 weeks and this translates into relatively small NNTs. By convention, NNTs are rounded up to the nearest whole number of patients. Rosuvastatin 10mg has an NNT of 4 compared to atorvastatin 10mg, 2 compared to pravastatin 20mg, and 4 compared to simvastatin 20mg (p<0.001, all comparisons). CONCLUSIONS: As a general rule, an NNT of 6 or less might be considered "good" for an acute treatment, while an NNT as large as 40 might be considered "good" for a chronic treatment (depending on the duration of treatment and the severity of outcome). In this context, an NNT of 4 for rosuvastatin 10mg compared to the next best in class (atorvastatin 10mg) would be considered a very favourable result.