THE childhood-onset spinal muscular atrophies (SMAs) describe a heterogeneous group of disorders that selectively affect the alpha motoneuron. We have shown that chronic childhood-onset SMA (SMA II and III) maps to a single locus on chromosome 5q (ref. 1). Acute SMA (SMA Type I/Werdnig-Hoffmann/severe/infantile) is the main cause of heritable infant mortality. Mapping the acute SMA locus by conventional methods is complicated by the rapidly fatal course of the disease and its recessive mode of inheritance. We present here the typing of four inbred acute-SMA families with DNA markers on chromosome 5q and analysis of these together with acute families from our previous study1 to demonstrate genetic homogeneity between the acute and chronic forms of SMA. The data indicate that the acute SMA locus maps to chromosome 5q11.2–13.3. Two families seem unlinked to 5q markers, raising the possibility of genetic heterogeneity or disease misclassification within the acute and chronic family sets.