The authors examined 47 subjects affected by acute phlebothrombosis of the lower limbs by means of light-reflection-rheography (LRR). The diagnosis was based on the results of clinical, Doppler, and duplex scanner evaluations. The results were compared with those obtained in 30 healthy subjects (control group). The LRR examinations were performed by two methods: the one consisting of a passive execution of postural movements of the limb, the other using the technique of plethysmographic venous occlusion. The pathological LRR curves were characteristic for each type of disease and for the site of thrombosis and were different from the normal ones. The results obtained show the usefulness of the method and it potential in studying the collateral circulation and the effects of different therapies.