The Cholesterol Treatment Trialists' Collaboration aims to provide reliable information about the effects on mortality and morbidity of treatments that modify blood lipid levels for a wide range of patient populations and risk groups. This protocol prospectively defines study eligibility, the main questions to be addressed, and statistical methods to be used. Additionally, by establishing a register of ongoing and planned trials prior to any trial results being known, this systematic overview attempts to avoid the methodologic problems and potential data-dependency of a retrospective project. The collaboration expects to have individual patient data on > 60,000 subjects by the year 2000, including 12,000 women and 20,000 elderly subjects, and should have good power to examine any effects on non-coronary artery disease events. Overall, there should be about 1,900 non-coronary artery disease deaths and > 2,000 total cancer events.