Ullrich's congenital muscular dystrophy (UCMD) is an autosomal recessive myopathy characterised by neonatal muscle weakness, proximal joint contractures and distal hyperlaxity. Mutations in the COL6A1, COL6A2 (21 q22.3) and COL6A3 (2 q37) genes, encoding the alpha 1, alpha 2 and alpha 3 chains of collagen VI, respectively, have been recently identified as responsible for UCMD in a total of 9 families. We investigated in detail the clinical and morphological phenotype of 15 UCMD patients from 11 consanguineous families showing potential linkage either to 21 q22.3 (6 families) or to 2 q37 (5 families). Collagen VI deficiency was confirmed on muscle biopsies or skin fibroblasts in 8 families. Although all patients shared a common phenotype, a great variability in severity was observed. Collagen VI deficiency in muscle or cultured fibroblasts was complete in the severe cases and partial in the milder ones, which suggests a correlation between the degree of collagen VI deficiency and the clinical severity in UCMD. No significant phenotypical differences were found between the families linked to each of the 2 loci, which confirms UCMD as a unique entity with underlying genetic heterogeneity.