Spinal muscular atrophy (SMA) is an autosomal recessive disorder, which causes degeneration of peripheral nerves and muscles. It usually presents in childhood due to an insufficient level of survival motor neuron protein. This is a case series of three children, who had SMA type 1 or 2 and were treated with nusinersen from the age of five months, 16 months, and five years, respectively. At one-year follow-up, all children had improved motor function, but the child, who was treated from the age of five months, had more pronounced motor improvements than the other children. In conclusion, nusinersen seems to improve motor development in SMA, and an early treatment start is crucial.