Objective In patients demonstrating dementia with Lewy bodies (DLB), pneumonia is a common complication. However, the prognostic factors for the survival time in DLB with pneumonia have not been investigated by autopsy in patients with neuropathologically confirmed DLB. Methods We conducted a retrospective study of the medical and autopsy reports of 42 patients admitted to a Japanese hospital between 2005 and 2014. The patients were neuropathologically diagnosed as having DLB by post-mortem examinations. We analyzed the effects of various factors on the time from DLB onset to death. Results Thirty-nine of the 42 patients with DLB (92.9%) developed pneumonia during hospitalization. The median age at DLB onset was 78 years and the median time from DLB onset to death was 8 years. The Cox proportional hazard model demonstrated cerebral infarction [Hazard Ratio (HR), 2.36 (95% CI 1.12-4.96), p=0.023], muscle weakness [HR, 2.04 (0.95-4.39), p=0.067], male sex [HR, 2.84 (1.24-6.50), p=0.014], and age at onset (≥78 years.) [HR, 4.71 (1.82-12.18), p=0.001] to be prognostic factors for a shorter time from DLB onset to death. Conclusion Careful treatment of cerebral infarction and muscle weakness of the lower extremities is crucial for DLB patients with pneumonia, especially for those over 78 years of age, in order to maximize the patients' life expectancies.