Congenital muscular dystrophies (CMD) are a very heterogeneous group of muscular disorders,recently classified in two types: 1 with and 2 without mental retardation. In the second group, merosine deficient congenital muscular dystrophy is included. It represents almost 40% of all CMD. The aim of the study was to describe seven patients with MD-CMD and the main clinical signs and motor progression of their disorder. We analyze seven patients with MD-CMD studied between 1999 and 2006. At moment of the study patients had ages ranging from 2 months to 13 years. Sex distribution was four males and three females. The biopsy analysis was made in Clínica Alemana, Calvo Mackenna Hospital, FLENI (two patients in each center) and Bambino Gesú Hospital in one patient. Results: There was no parental consanguinity nor neuromuscular disorders in the family histories. All seven patients had severe hypotonia,muscular weakness and areflexia since the neonatal period. All had leucodystrophic changes in cerebral white matter,high levels of creatinkinase (CK) ranging 700-43.000 U/L, myopathic EMG, and developed contractures at elbows and knees. All patients had hip dislocations, three presented seizures, three had peripheral neuropathy, and one became ventilator-dependant. Only one developed independent gait but became wheelchair-dependant by 11 years of age. Cognitive level was normal in all patients. The muscular biopsy was open in two patients, needle biopsy in four and skin biopsy in one patient. Merosine inmunohystochemistry was made with 300 kDa monoclonal antibodies in 6, and additional 80 kDa antibodies in 2. All patients had total merosine deficiency in the analyzed tissue. Patients with total merosine deficiency present severe hypotonia and muscular weakness since neonatal period,and have little probabilities of obtaining independent gait.