Mutations in the tail of the cytoplasmic dynein molecule have been reported to cause neurodegenerative disease in mice. The mutant mouse strain Legs at odd angles (Loa) has impaired retrograde axonal transport, but the molecular deficiencies in the mutant dynein molecule, and how they contribute to neurodegeneration, are unknown. To address these questions, we purified dynein from wild-type mice and the Legs at odd angles mutant mice. Using biochemical, single-molecule, and live-cell-imaging techniques, we find a marked inhibition of motor run-length in vitro and in vivo, and significantly altered motor domain coordination in the dynein from mutant mice. These results suggest a potential role for the dynein tail in motor function, and provide direct evidence for a link between single-motor processivity and disease.