Invasive procedures to revascularize occluded blood vessels rely on the mechanical response of the diseased tissue. Failure rates associated with such procedures show the need for improvement. to understand the associated mechanics, the material properties of atherosclerotic plaque should be known; yet data are scant. The purpose of this study was to investigate the different mechanical responses exhibited by plaques with different compositions, focusing specifically on radial compressive behavior. A custom-built experimental system was developed that was fully computer controlled with a broad range of loading capabilities. A temperaturecontrolled, physiologic specimen bath allowed testing at 37ยฐC. Monotonically loaded specimens showed that plaque behavior was nonlinear under finite deformations. A multiple cycle protocol, executed in two phases, distinguished three types of mechanical response of different plaques. The differences in behavior were associated with histologic differences in plaque composition, and mechanically characterized by different ''repeatability'' (the stabilization of the cyclic response) and ''recoverability'' (the second loading phase retracing the first loading phase behavior). Type 1 behavior was categorized by repeatability and recoverability. Type 2 behavior displayed repeatability but only partial recovery during the second loading phase. Recovery was absent in type 3 behavior. The histologic observations demonstrated that calcified tissue was present only in specimens displaying type 1 behavior. Fibrous tissue and part of a modified media (due to disease) were present in specimens displaying type 2 behavior. An atheroma, along with a relatively thin modified media, was present in specimens displaying type 3 behavior. The differences in the maximum stretches attained at the end of phase I loading, the stretch offset from the first to the 15th cycle of phase I loading, and the hysteresis in the first and 15th cycles of phase I loading distinguished the specimen behaviors with statistical significance. These compression data showed that plaques exhibit composition-and history-dependent nonlinear and inelastic responses under finite deformations.