Passive-cavity transverse-mode stability properties such as the transverse-mode discrimination ratio and aberration sensitivity are analyzed. The influence of the diffractive formation of the transverse-mode field distribution on the aberration sensitivity is studied, along with the relationships between this sensitivity measure and the diffractive transverse-mode discrimination in a cavity. It is shown that the passive unstable-cavity mode stability properties are connected with the properties of the central core region of the resonator that is defined by the cavity Fresnel zone structure. Specifically, the transverse-mode discrimination or separation ratio with any perturbation or aberration source is a maximum when there is a constructive interference along with the cavity optic axis between the edge scattered converging wave field from the feedback aperture and the dominant geometric mode of the cavity.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.