To determine the value of diffusion weighted imaging (DWI) in the diagnosis of hereditary spinocerebellar ataxia 3 and the Machado Joseph disease (SCA3/MJD) and hereditary spastic paraplegia 4 (SPG4).We scanned 13 patients with SPG4, 30 patients with SCA3/MJD (21 onset patients and 9 with only genetic abnormalities), and 27 healthy volunteers with DWI. The processing data were apparent diffusion coefficient (ADC). The above data were grouped for comparative study.In the precentral gyrus, posterior limb of the internal capsule, cerebral peduncle, pons, cerebellar cortex and cerebellar white matter, the ADC of onset SCA3/MJD patients increased compared with the control group. The ADC of non-onset SCA3/MJD patients increased only in the cerebellar dentate nucleus compared with the control group. In the cerebellar cortex, the ADC of onset SCA3/MJD patients was significantly higher than the non-onset SCA3/MJD. The ADC of onset SCA3/MJD patients was significantly higher in the posterior limb of the internal capsule, cerebellar cortex, cerebellar white matter and pons than that of SPG4 patients. In the precentral gyrus, the ADC of SPG4 was significantly higher than control.DWI is useful in the diagnosis of SCA3/MJD and SPG4.