Acute infantile spinal muscular atrophy (SMA type I, Werdnig-Hoffmann disease) has generally been accepted as an autosomal recessive disorder. However, several investigators have noted a slightly increased male to female ratio. We describe here a family with two affected male sibs who had a form of acute infantile SMA with congenital bone fractures, whose parents were first cousins. Pedigree analysis strongly suggested autosomal recessive inheritance, but X linked recessive inheritance could not be ruled out. In view of the heterogeneity of the SMAs, and the distinct clinical features found in our patients, we suggest that their infantile SMA might well be a distinct entity. We suggest that SMA I with congenital contractures and bone fractures appears to be a recognisable disorder that can be distinguished from the more common classic form of SMA I.