The characteristics of hyperventilation syndrome (HVS) were studied in 508 patients who visited our hospital over 11 years. Information regarding symptoms and laboratory data was collected from the clinical records, and outcome was surveyed with a questionnaire mailed to all patients. Patients with acute HVS ranged in age from 5-85 years, and acute HVS was particularly prevalent among women in their late teens. Triggers of HVS included anxiety, nausea & vomiting, and fever due to the common cold. The primary symptoms were dyspnea and numbness, but these differed from the symptoms that appeared during a provoked attack, Half of the patients had no underlying disorder, but the others were suffering from neurosis, cardiovascular disorders, or other diseases. These characteristics of acute HVS did not differ from those seen in patients in whom the diagnosis of HVS was confirmed with arterial blood gas analysis. Half of the patients recovered without treatment, and the others underwent paper-bag rebreathing or intravenous infusion of sedatives. The prevalence of chronic HVS was 2% and almost all those patients were middle-aged women. In contrast, the questionnaire revealed that half of the patients had repeated HVS attacks. In 10% of the patients, these attacks persisted for more than 3 years. Many of these patients reported that they sighed frequently and felt air hunger while in remission. These findings were compatible with the criteria for chronic HVS. Therefore, it may be possible to diagnose HVS from symptoms alone, without hyperventilation provocation tests. In conclusion, these data underscore the importance of clinical symptoms in the diagnosis of HVS.