Proximal spinal muscular atrophy (SMA) is classified into three main subtypes (I-III), defined by age at onset and achieved motor milestones. As age at onset can be very early in SMA II and III (IIIa, onset < 3 years) and does not necessarily correlate with prognosis, the question arises whether the child can be correctly assigned to a specific SMA type at the time of presentation based on the assessment of motor function. Therefore we studied the motor milestones in 175 SMA type II and 266 SMA type III patients. In SMA II, 73% of the patients sat within the normal age range (up to 9 months), the remainder learned to do so at ages between 10 and 30 months. In SMA III, the walking milestone was passed with delay (given an upper normal limit of 18 months) in 10% of all and 16% of SMA IIIa patients (median age 13 months, range 9-53 months). There was a correlation between late sitting and walking in SMA III, since those who sat after 9 months were responsible for the majority of delayed walkers. The median age when becoming chairbound did not differ between early-onset SMA III patients who walked with delay and those who walked within the normal age range (10.2 versus 10.5 years). In conclusion, a significant proportion of patients with early-onset SMA classified as SMA II on the basis of achieved motor function turned out to be SMA III at later follow-up. It is important to reassess a child in the first 2-4 years, to determine whether walking can be achieved with or without aids, as children who start to walk late have a similar favourable outcome for ambulation compared to earlier walkers.