['In laser Doppler measurements, perfusion values averaged over different and basically unknown path lengths are recorded. To facilitate quantitative path length resolved perfusion measurements, we developed a phase modulated Mach-Zehnder interferometer with spatially separated fibers for illumination and detection. The goal of this study is to measure in vivo optical path lengths and path length resolved Doppler shifts and to compare these with conventional laser Doppler perfusion measurements.', 'With a phase modulated Mach-Zehnder interferometer, we performed path length resolved perfusion measurements on human skin and its variations to external stimuli and compared these with conventional laser Doppler perfusion measurements. The method was evaluated in three human subjects on the dorsal side of the forearm to establish inter-individual within-site variations. Measurements were performed at three different locations of one individual for observing intra-individual inter-site variations resulting from the heterogeneity of the tissue, both in the static matrix and in the microvascular architecture of the skin. In all measurements, perfusion was simultaneously measured with a conventional laser Doppler perfusion monitor.', 'In this study, we show the first results of path length resolved perfusion measurements in skin and its variations to occlusion and Capsicum cream provocation. From our data, we deduced the Doppler shifted fraction of photons, which is related to the blood volume, and the path length dependent average Doppler shift, which is related to the mean velocities of red blood cells. The Doppler shifted fraction of photons is decreased from 28% to 18% during occlusion and increases to 41% when capsicum cream was applied to the skin. Inter- and intra-individual inter-site measurements demonstrated variations in optical path length distributions and path length resolved Doppler shifts. The Doppler shifted fraction of photons measured on the fingertip is about 38% and that measured on the dorsal and palmar sides of the forearm are 32% and 17%, respectively. The path length distributions depend on the skin site that is being probed and the intra-individual inter-site variability is higher than the inter-individual within-site variability measured on comparable sites between different individuals.'] Do in vivo optical path lengths and path length resolved doppler shifts of multiply scattered light?