Respiratory health problems are one of the main causes of morbidity and mortality in adult people with cerebral palsy (CP). The influence of respiratory muscle training has not yet been studied in this population group. The objective of the study was to evaluate and compare the efficacy of two protocols with inspiratory muscle training (IMT), low intensity and high intensity, to improve respiratory strength and pulmonary function in adults with CP. The study was a controlled, randomized, double-blind trial with allocation concealment. Twenty-seven institutionalized CP patients were recruited and randomly distributed in the high-intensity training group (HIT) or low-intensity training group (LIT). Over 8 weeks, an IMT protocol was followed 5 days/week, 10 series of 1min with 1min rest between them. HIT trained with a load of 40% of the maximum inspiratory pressure (MIP) and LIT with 20%. Respiratory strength and pulmonary function were evaluated. After IMT intervention, MIP, maximum expiratory pressure, forced expiratory volume in 1s (FEV1) and peak expiratory flow increased in both groups; in HIT 29%, 19%, 13%, and 8%, respectively (p=0.000, p=0.000, p=0.002, p=0.001) and in LIT 17%, 7%, 3%, and 4%, respectively (p=0.000, p=0.000, p=0.049, p=0.113). All the improvements were significantly higher in HIT than in LIT. Inspiratory muscle training improved respiratory muscle strength and pulmonary function in adults with CP. Training with a 40% MIP load improved all the evaluated parameters and was the most effective treatment for adults with CP.