[Purpose] We aimed to determine the relationship between gait speed and diaphragm thickness in community-residing patients with hemiplegia caused by cerebrovascular accidents. [Participants and Methods] We recruited 11 elderly participants (six male and five female, mean age 71.1 ± 13.6 years) from an outpatient rehabilitation unit. The inclusion criteria were as follows: patients with hemiplegia caused by cerebrovascular accidents, those able to walk without assistance, and those able to understand our instructions. We measured the diaphragm thickness on both the paretic and non-paretic sides in each participant during maximum exhalation and inhalation during three laboured breaths by ultrasonography with a 7.5-MHz linear scanner. The liner scanner was placed on the eighth or ninth rib between the anterior and middle axillary lines. And their gait speed was measured during a 10 m walk. [Results] There was a strong positive correlation between gait speed and the ratio of diaphragm thickness between the paretic and the non-paretic sides during maximal inspiration. The other measured parameters did not show significant correlation with gait speed. [Conclusion] The symmetrical thickness of the diaphragm is a key factor in increasing gait speed in patients with hemiplegia. These findings may contribute to the development of trunk muscle strength-training programs that improve trunk function and gait speed in patients with hemiplegia.