The efficacy and safety of simvastatin were evaluated in an open multicenter study over a 24-week period. One hundred seventy-two patients (91 men, 81 women) with primary hypercholesterolemia (mostly polygenic) were enrolled in 14 Centers in Northern Italy. The mean age was 55.8 +/- 9.7 years and the mean baseline total cholesterol level was 305 +/- 59 mg/dL. After 4 weeks on an AHA step 1 diet, patients who met the inclusion criterion (total cholesterol > or = 250 mg/dL) were given simvastatin 10 or 20 mg in the evening. The dose could be titrated up to a maximum of 40 mg o.d. at week 6 and 12. No dose titration was allowed after week 12. One hundred forty-nine patients (86.6%) completed the study according to the protocol, 2 (1.2%) were withdrawn from the study because of adverse events not related to the drug, 21 (12.2%) were unavailable for follow-up. Simvastatin treatment was associated with a sustained dose-related reduction in total and LDL cholesterol (-28% and -39% respectively at the end of the study). Triglycerides showed a significant descending trend (-16% at week 24) and HDL-C increased by 9%. Apolipoproteins were measured in only 25 patients: apo B was reduced by 30% and apo A1 increased by 9%. Clinical side effects were not relevant. Mean levels of GOT, GPT and CPK significantly increased after 6 weeks on simvastatin, but remained stable and at any rate ioitlin the normal range thereafter. Eight patients (5.4%) experienced small transaminase level elevations (< 3ULN) and six (4%) small CPK elevations.(ABSTRACT TRUNCATED AT 250 WORDS)