In this study we examined the deletion of SMN and NAIP genes in 60 Tunisian families. There were 35 patients with type I SMA. 18 with type II SMA. 6 with type III SMA and I with type IV SMA. The age of onset was before 6 months for type I, between 6 months and 2 years for type II, between 2 years and 17 years for type III and 30 years for type IV. Exon 7 of SMNI gene was homozygously deleted in 95% (57/60) of SMA patients. There was a higher frequency of homozygous absence of SMN1 in type I and type II (100% and 94% respectively) than in type III (66,7%). SMN1 exon 8 was undetectable in 88% (53/60) of patients. The case type II patient with homozygous deletion of SMNI exon 7 and not exon 8 was tested for the presence of a hybrid SMN gene. This patient showed in the second PCR a SMN1 exon 8 product by restriction site assay indicating that a gene conversion event had occurred. All parents' individuals retained one copy of their SMN1 gene. Exon 5 of NAIP gene was homozygously deleted in 58% (35/60) of patients (77% in type I (27/35), 27,7% in type II (5/18), 50% (3/6) in type III. No patient had a deletion in NAIP gene without a deletion in the SMN1 gene. Homozygous deletion of NAIP exon 5 was detected in 1 parent. Our results show that the incidence of NAIP deletion is higher in the more severe SMA cases.