Background Medically unexplained dyspnea refers to a condition characterized by a Sensation of dyspnea and is typically applied to patients presenting with anxiety and hyperventilation without underlying cardiopulmonary pathology. We were interested to know how anxiety triggers hyperventilation and elicits subjective Symptoms in those patients. Using an imagery paradigm, we investigated the role of fearful imagery in provoking hyperventilation and in eliciting Symptoms, specifically dyspnea.Methods Forty patients with medically unexplained dyspnea and 40 normal subjects matched for age and gender were exposed to scripts and asked to imagine both fearful and restful scenarios, while end-tidal PCO2 (PetCO2) and breathing frequency were recorded and subjective Symptoms evaluated. The subject who had PetCO2 falling more than 5 mmHg from baseline and persisting at this low level for more than 15 seconds in the imagination was regarded as a hyperventilation responder.Results In patients with medically unexplained dyspnea, imagination of fearful scenarios, being blocked in an elevator in particular, induced anxious feelings, and provoked a significant fall in PetCO2 (P＜0.05). Breathing frequency tended to increase. Eighteen out of 40 patients were identified as hyperventilation responders compared to 5 out of 40 normal subjects (P＜0.01). The patients reported Symptoms of dyspnea, palpitation or fast heart beat in the same fearful script imagery. Additionally, PetCO2 fall was significantly correlated with the intensity of dyspnea and palpitation experienced during the mental imagery on one hand, and with anxiety Symptoms on the other.Conclusions Fearful imagery provokes hyperventilation and induces subjective Symptoms of dyspnea and palpitation in patients with medically unexplained dyspnea.