The method to determine red blood cell deformability by laser diffraction is presented. It combines ektacytometry (laser diffraction) with image analysis. Laser light is passed through erythrocyte suspensions, which are sheared in a Searle like viscometer. The laser diffraction patterns are photographed by a CCD camera and further analyzed by a computer. Flexibility is characterized by the quotient of minor and major axes of isointensity lines of the elliptically transformed diffraction patterns. The variation coefficient of the measurement is less than 0.5%.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.