We reviewed the natural history and assessed the SMN2 copy number of 66 patients with infantile spinal muscular atrophy (SMA) type I born between 2000 and 2005 in Germany whose diagnosis was confirmed by a homozygous SMN1 deletion in the first 6 months of life. After excluding patients who had received valproic acid, the median/mean age at disease endpoint was 6.1/7.3 months (range 0.0–34.0). Four (6.1%) patients with one SMN2 copy had severe SMA type ‘0’ with joint contractures and respiratory distress from birth. Median/mean age at onset (months) in 57 (86.3%) patients with two SMN2 copies was 1.2/1.3, and 3.5/3.4 in 5 (7.6%) patients with three SMN2 copies. Median/mean age at disease endpoint was 6.5/7.8 months (range 0.5–30) in patients with two SMN2 copies. All patients with three SMN2 copies were still alive at 10–55 months, two of them under permanent ventilation. Our data are relevant for prognostication and genetic counselling. The observed clinical variability, especially in the group with two SMN2 copies, might be important for clinical trials in SMA I where a possible control group could be defined as follows: age at onset within 4–5 months, age at genetic diagnosis <6 months, two SMN2 copies present, head control in less than 10%, no respiratory distress from birth, disease endpoint either age at death or age at permanent ventilation.