Oral propranolol (OP) has been shown to be effective in the treatment of complicated infantile hemangiomas (IHs), but optimal treatment duration to avoid relapses after stopping OP treatment has not been established. The objective of this study was to compare the frequency of relapses in long-term OP treatment with that of short-term OP treatment. This was a retrospective cohort study of 30 patients with complicated IHs who received treatment with OP. Patients were divided into two groups: OP treatment of 8 months or less and OP treatment of longer than 8 months. OP was started at 1 mg/kg/day in three doses every 8 hours for 1 week and increased to 1.5 to 4 mg/kg/day afterward. Ultrasound was used to objectively measure the response to treatment. Clinical and ultrasound assessment showed a decrease in IH size and resolution of complications in all patients (n = 30). In the short-term group (n = 10), nine patients (90%) relapsed after stopping treatment. In the long-term group (n = 20), the duration of treatment was 12 months in all patients, and only 1 patient out of the 20 treated (5%) showed relapse 2 months after finishing the full treatment (odds ratio = 18, 95% confidence interval 2.6, 123, p < 0.001. Twelve months of treatment of IH with OP is associated with a significantly lower rate of relapse than with shorter treatment.