OBJECTIVES: Montelukast 4mg (MTL-4) is a recent add-on therapy for young asthmatic children. French regulators have requested real-world evidence on effectiveness of MTL-4 in infants (6-24 months), compared to inhaled corticosteroid (ICS) therapy. National claims data (SNIIR-AM) are now available to public investigators. SNIIR-AM records exhaustive medical resource utilization of the French population, i.e. 65.4 millions. Due to the limited population of infants exposed to MTL-4 from 2010 (i.e. 78 000 children 6-24 months old), SNIIR-AM represents a good tool to investigate its effectiveness. We first tested the feasibility of such a study in a pilot phase conducted on a 1/97th representative sample of the full data set (EGB: Representative Sample of Beneficiaries), to validate identification and evaluation criteria, and to identify potential pitfalls in the methods or insufficient statistical power for groups comparison, in order to take them into account in the finalized version of the protocol. We present hereafter the main conclusions of the pilot project. METHODS: We preselected infants receiving ≥ 2 consecutive dispensations of any respiratory drugs (R03 ATC classification) and presenting an initial exacerbation within 6 months following the first dispensing. Asthma exacerbation was identified by asthma-related hospitalizations, dispensing of oral corticosteroids, addition of short-acting beta agonist to existing respiratory therapy, switch to an ICS therapy with higher dosage, or switch to nebulized CS.  RESULTS: Our sample included 1,149 infants (mean age 13 months, 64% boys). Among them, 51 and 768 were assigned to Montelukast and ICS groups, respectively. Infants with a exacerbation during the 6 months post inclusion were 78.8% and 78.4% in each group, respectively (51% and 62% for oral costicosteroids only). CONCLUSIONS: The results of this pilot study support the feasibility of our SNIIR-AM project, regarding inclusion criteria and identification of outcomes. These data allowed us to finalize the SNIIR-AM study protocol that is ongoing.