A method is described which is of assistance in diagnosing Raynaud's phenomenon by making it possible to estimate the degree of cold sensitivity. 5-minute ischemia provoked by tourniquet round the wrist results in reactive finger hyperemia when the tourniquet is released. In 25 of 28 patients with Raynaud's phenomenon, postischemia reactive hyperemia was suppressed by cooling the hand to between 10 and 20 degrees, but remained unchanged in normal subjects. The critical temperature at which Raynaud's phenomenon occurs can be determined in a given patient, and the natural history of the disease and the effect of treatment can be observed. Cold induced reactive hyperemia suppression is probably caused by Burton's critical closing phenomenon.