Spinal deformity in the form of kyphosis or kyphoscoliosis occurs in most patients with Duchenne Muscular Dystrophy (DMD), a fatal X-linked disorder caused by an absence of the sub-sarcolemmal protein dystrophin. Mdx mice, which also lack dystrophin, show thoracolumbar kyphosis which progresses with age. We hypothesize that paraspinal and respiratory muscle weakness and fibrosis are associated with the progression of spinal deformity in this mouse model, and similar to DMD patients, there is evidence of altered thoracic conformation and area. We measured kyphosis in mdx and age-matched control mice by monthly radiographs and the application of a novel radiographic index, the kyphotic index (KI), similar to that used in boys with DMD. KI became significantly less in mdx at nine months of age (3.58 ± .12 compared to 4.27 ± .04 in the control strain, P<0.01), indicating more severe kyphosis, and remained less from 10-17 months of age. Thoracic area in 17 month old mdx was reduced by 14% compared to control mice (P<0.05). Peak tetanic tension (P_o) was significantly lower in mdx; falling 47% in old mdx latissimus dorsi muscles, 44% in intercostal strips, and 73% in diaphragm strips (P<0.05). Fibrosis of these muscles and the longissimus dorsi, measured by hydroxyproline analysis and histological grading of picrosirius red stained sections, was greater in mdx (P<0.05). We conclude that KI is a useful measure in mdx and other kyphotic mouse strains, and assessment of paralumbar and accessory respiratory muscles enhance understanding of spinal deformity in muscular dystrophy.