The proximal spinal muscular atrophies (SMA) are a clinically and genetically heterogeneous group of neuromuscular disorders which are characterized by selective degeneration of motor neurons in the spine and brainstem. The clinical features resemble other muscle diseases, the diagnostic criteria of proximal SMA have recently been defined by the International SMA Consortium. The classification of the clinical picture in different subgroups is still a focus of discussion. At present it seems likely that childhood onset SMA represents a broad spectrum of various ages of onset and different degrees of disability. Apart from different modes of inheritance, there is further evidence of heterogeneity in proximal SMA. The autosomal recessive forms represent the second frequent recessive disorder after cystic fibrosis, whereas autosomal dominant inheritance is an exception in childhood onset SMA. There are no convincing reports of X-linked SMA hitherto. With mapping of acute and chronic forms of childhood SMA to chromosome 5q11.2-13.3 diagnosis by use of DNA markers in affected families has become available. The current possibilities but also problems and limitations of genotype analysis are discussed, with special regard to the application of prenatal diagnosis. We report on the first experiences with prenatal diagnosis in 37 SMA families.