Asymmetry of light backscattering from a shear flow of 1 mm thick layer of whole blood was measured. The blood samples from different health and diseased individuals were placed in a gap of a cylindrical Couette cell with inner cylinder rotating with variable rates. Probing was performed with a CW He-Ne laser (633 nm). The difference in intensities of backscattered light detected through a transpatent wall of the stationary outer cylinder with a photodetector located at variable distances from the illuminating beam along and opposite to the flow directions depends on the distance between the illuminating and detecting probes, on shear rate, on hematocrit, and on the type of disease. The experiments conducted with blood samples from patients suffering from heart disease, bronchial asthma, and rheumatoid arthritis showed stable and reproducible difference in the amount of asymmetry. This makes the technique potentially applicable for optical biomonitoring and, also, for the study of rheology of concentrated suspensions of large deformable particles, like erythrocytes.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.