Canine hip dysplasia is the developmental orthopedic disease with highest incidence in dogs. There are many studies on this subject, but there is not much information about the correlation between clinical signs, radiological findings and microscopic morphological alterations involving the pectineus muscle in the pathophysiology of hip dysplasia. We evaluated the histopathological alterations of pectineus muscles in patients diagnosed with hip dysplasia and their correlation with clinical signs, degree of hip dysplasia and pathophysiology of the disease. Eleven dogs between 1–10 years of age, of various breeds, diagnosed with hip dysplasia were selected. The diagnosis was based on history, physical examination, clinical signs and radiographic analysis with measurement of the angle of Norberg. The degree of hip dysplasia and the hip joint alterations were based on the classification proposed by the Fédération Cynologique Internationale (FCI) and Orthopedic Foundation for Animals (OFA). Biochemical parameters and CBC were determined as normal in all dogs. Patients underwent pectinectomy as the only treatment. Once pectinectomy was made, the occurrence of histopathological changes was evaluated, then proceeded to characterization of lesions. Atrophy and histopathological changes in the pectineus were presented in all dogs. High correlation was found between the degree of radiological dysplasia and degree of morphological alteration of the muscle. The results are indicative that pectineus atrophy is highly involved in the pathophysiology of hip dysplasia in dogs, and suggests the pectineus atrophy as an etiologic factor of hip dysplasia.