A semiconductor laser speckle velocimeter that uses the self-mixing effect is studied for noninvasive relative blood flow measurements. The random modulation of intensity and spectra of the laser diode caused by the backcoupling of the scattered light from the red blood cells into the laser cavity is detected as a speckle signal with a photodiode inside the laser package. The autocorrelation of this self-mixing speckle signal gives information on the flow velocity of the blood. The proposed method is elucidated with in vitro and in vivo experiments. The results of these measurements are given together with a discussion of dependence of speckle signal of a self-mixing laser diode (SMLD) on various parameters such as velocity, hematocrit level of blood, and background reflectance of blood suspension. © 2000 Society of Photo-Optical Instrumentation Engineers.