We clarified the problems in respiratory management in patients with phrenic nerve palsy (PNP) after open heart surgery. From December 1988 to March 1991, 248 adult patients underwent open heart surgery with topical myocardial cooling. In these patients PNP was diagnosed in 17 patients (6.9%). Age of these patients at operation ranged from 25 to 76 years with a mean age of 57 years. 9 patients were valvular heart disease (7 were reoperation), 7 were coronary artery disease, and one patient was aortic aneurysm. PNP was diagnosed by chest roentgenogram or percutaneous phrenic nerve stimulation test (PNST). Frequency of long-term intubation (> 14 days) was higher in complete PNP patients than incomplete PNP patients. 2 patients, who were observed effort dyspnea at the time of respiratory weaning, were extubated followed by respiratory and circulatory deterioration, and were reintubated. PNP were confirmed after reintubation by PNST. These patients died of pneumonia. Another 2 patients were observed effort dyspnea during respiratory weaning, and were doubted of PNP. PNP were confirmed by PNST before extubation. After hemodynamic stabilization and respiratory physiotherapy, these patients were extubated without any trouble. One patient, who required repeated intubation, was diagnosed PNP by PNST. This patient was extubated after confirmation of improvement of PNP by follow-up PNST. The incidence of PNP was higher in patients with previous open heart surgery than without previous operation. We supposed that the dissection around the heart might be one of major cause of postoperative PNP. So, in recent 4 cases of mitral reoperation, we attempted to reach mitral valve through atrial septum with minimum dissection.(ABSTRACT TRUNCATED AT 250 WORDS)