Primary pulmonary hypertension is a disease with a high mortality rate and for which there is no satisfactory medical treatment. The safety of long-term inhalation of nitric oxide (NO) as a treatment is described. A 9-year-old girl inhaled NO for 32 weeks, accompanied with oral administration of beraprost sodium. Although NO did not improve her long-term prognosis, it eased the patient's dyspnea and increased her blood oxygenation. At doses of 20 ppm or more, attempts to withdraw from inhaled NO seemed to lead to an immediate elevation of the pulmonary artery pressure. This rebound phenomenon did not happen at doses under 5 ppm. This case study suggests that long-term inhalation of NO is safe and effective, but that pulmonary hypertension may rebound following withdrawal at higher doses of NO.