The records of 103 male and 39 female patients with unexplained diaphragmatic paralysis were reviewed. A probable cause of the paralysis was not revealed by the initial history, physical examination, or review of plain chest roentgenograms. Paralysis occurred on the left in 82 patients (58%), on the right in 58 (41%), and bilaterally in two (1%). Initially, 64 patients (45%) had symptoms; dyspnea, cough, and chest wall pain were the most common. Long-term follow-up showed the best prognosis to be for patients with chest wall pain and cough (improvement in 82% and 78%, respectively); dyspnea improved in only 34% of patients with this complaint. Intrathoracic malignant lesions with phrenic nerve involvement were subsequently diagnosed in five patients (3.5%) and progressive neurogenic atrophy in one (0.7%). Roentgenographic follow-up showed return of normal diaphragmatic position in only 12 instances (9.2%). Patients with unexplained diaphragmatic paralysis are unlikely to have an underlying occult malignant or neurologic process, but recovery of diaphragmatic function is also unlikely and subsidence of related symptoms is variable.