Parkinson disease (PD) is the second most common neurodegenerative disease of adult onset. Is a progressive movement disorder including tremor, bradykinesia, rigidity and postural instability, with an age onset between 43 and 66 years. Histopathologically, is characterized by a severe loss of dopaminergic neurons in the substantia nigra and inclusions consisting of insoluble protein aggregates called Lewy bodies, this are comprised in part of alpha-synuclein. The etiology of PD is still not fully understood, but genetic analyses, epidemiologic studies and experimental models of PD are providing important new insights into the pathogenesis of PD. To determine allelic and genotypic frequencies of polymorphism IVS4+66A-G in the alpha-synuclein gene and to demonstrate its association with PD in northwest Mexican population. Genomic desoxyribonucleic acid (DNA) from 51 PD patients and 121 persons without PD were achieved by polymerase chain reaction and analyzed the allelic and genotypic distribution in IVS4+66A-G polymorphism of alpha-synuclein gene. The genotypic frequency of IVS4+66AA was 43.1% in PD patients and 38.8% in control group; IVS4+66GG was 2% in PD patients and 4.1% in control group, whereas 54.9% in PD patients and 57.1% in control group were heterozygous. Statistical differences were not observed between groups (p