During the course of a study of skin blood flow under applied external pressure, it became apparent that decreasing blood flow by loading the skin surface reveals problems that are fundamental to the method of laser Doppler flowmetry. These problems have to do with the fact that the laser Doppler is extremely sensitive to red cell motion of any kind, whether associated with the ordered red cell motion of blood flow or the random red cell motion associated with changes in temperature or vessel occlusion. This effect becomes increasingly important whenever blood flow is compromised (a situation of considerable clinical significance), since the random portion of the signal then becomes significant in comparison with the diminished blood flow. Experiments have been conducted in living animals and with stationary drops of blood which clearly show the importance of these effects with regard to the interpretation of laser Doppler signals. Significant laser Doppler flow signals were repeatedly observed after manipulations which could reasonably be expected to reduce blood flow to zero.