Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by mutations of the SMN1 gene. It is characterized by significant phenotype variability. In this study, we analyzed possible phenotype modifiers of the disease - the size of the deletion in the SMA region, the number of SMN2 gene copies, as well as the effect of gender. Among the factors analyzed, two seem to influence the SMA phenotype: the number of SMN2 gene copies and a deletion in the NAIP gene. A higher number of SMN2 copies makes the clinical symptoms more benign, and the NAIP gene deletion is associated with a more severe phenotype. The influence of gender remains unclear. In a group of 1039 patients, 55% of whom were male, the greatest disproportion was in the SMA1 (F/M = 0.78) and SMA3b (F/M = 0.45) forms. In SMA1 a deletion in the NAIP gene was seen twice as frequently in girls compared to boys. In three patients, we observed genotypes atypical for the chronic forms of SMA: two patients with SMA3a and 3b had a deletion of the NAIP gene, and a third patient with SMA2 had one copy of the SMN2 gene.