Pulmonary arterial hypertension (PAH) is commonly associated with the CREST (calcinosis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) syndrome. Inhaled nitric oxide (iNO) is often used to assess acute vasoresponsiveness in patients with PAH, and reports of adverse reactions have been infrequent. We describe two of nine patients with PAH and CREST syndrome who had pulmonary edema develop during acute iNO testing. This complication was not encountered in the 46 patients with other forms of PAH tested with iNO. We suggest that iNO should be used with caution, if at all, to test acute vasoreactivity in patients with CREST syndrome.