Twenty-five patients with Raynaud's phenomenon had serial cold challenges during a double blinded drug trial. The data were analyzed to determine the reproducibility of cold provocation in the induction of critical closure of the digital artery in patients with Raynaud's phenomenon. Finger systolic pressure (FSP) was measured after local digital cooling using a digital strain gauge placed around the distal phalanx. Nineteen of 25 patients completed the study. The prevalence of inducing a Raynaud's attack decreased with each successive cold challenge from 74% of patients at initial challenge to 42% at the 3rd challenge. A lower temperature was required to induce a Raynaud's attack at last challenge (10.6 +/- 0.6 degrees C) compared to the first cold challenge (13.2 +/- 1.0 degrees C). Our data demonstrate adaptation to a laboratory cold challenge through the winter months in patients with Raynaud's phenomenon and show it is an important factor in objectively assessing drug efficacy in the treatment of Raynaud's phenomenon.