OBJECTIVE: This study examined the prevalence of deletion and subtle mutations of survival motor neuron (SMN) gene in children with spinal muscular atrophy(SMA). METHODS: Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was used to detect the deletion of SMNt exon 7 in 25 children with SMA (type I 5 cases, type II 3 cases, and type III 17 cases) and in 24 healthy relatives of these patients. SMA was diagnosed clinically and pathologically. The subtle mutations of SMN in encode regions were screened by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) combined with DNA direct sequencing in the patients without SMNt deletion and their relatives. RESULTS: Deletion of exon 7 of the SMNt gene was found in 5 cases of SMA type I(100%), 3 cases of type II (100%) and 6, type III(35%). No subtle mutation of SMN was found in encoded regions in 11 cases of type III SMA without SMNt deletion. The 24 relatives of SMA patients did not show the deletion and subtle mutation of SMN. CONCLUSIONS:  ①Detection of SMNt gene exon 7 deletion can be recommend as a definitive diagnostic method for SMA, and showed promise to replace invasive examinations, such as electromyogram and muscular biopsy. ②In patients with type III SMA without SMNt deletion, diagnosis is still made clinically. ③No subtle mutation of SMN was found in SMA type III patients without SMNt deletion, suggesting genetic heterogenicity might exist.