Introduction Sirolimus has become the third leg in the treatment of vascular anomalies, initially relegated as rescue therapy for refractory lesions but lately recommended as first line. Our objective is to determine when it is best to start sirolimus treatment. Methods A retrospective review of patients with vascular anomalies treated with sirolimus during a 9-year period was performed. The variables analyzed included subtype of vascular anomaly, age at the start of treatment, sirolimus dosage and levels, response and duration of treatment among others. Results 129 patients were included and classified according to ISSVA in: tumor (7), lymphatic malformation (60), venous malformation (16), combined malformations (26), others (22). Overall response was positive in 90.8%. Median duration of treatment was 5.5 months (0-110). Response was not influenced by dosage and levels, neither by subtype of vascular anomaly, location, extension or symptoms. However the lower the age of starting sirolimus, the better the response, mainly under 5 years of age (p=0.004). At 6 months 67% of patients had responded while at 12 months over 84% did. By age, patients under 5 years showed positive response at a median time of 2 months, compared to 5 and 7 months from older patients. Time until a positive response was not influenced by subtype neither by severity. Conclusion Overall response to sirolimus was good and most patients responded irrespective of their severity in location, extension or symptoms. Patients under 5 years old respond better and faster making our goal to attempt for an early treatment.