Objectives-Failure to thrive (FTT) in infants is characterized by growth failure. Although, cow's milk allergy (CMA) may have an impact on growth and leads to FTT, data are still limited. We focused on FTT as a possible clinical marker for an early diagnosis of CMA. The aim of the present study was to evaluate the implications of cow's milk hypersensitivity in infants with FTT and the growth catch-up after a cow's milk-free diet (CMFD). Methods-A cross-sectional study of all consecutive infants evaluated at the Pediatric Nutrition and Allergy Unit of the University Hospital of Bari (Italy) from January 2016 to April 2018 with a medical-driven diagnosis of FTT. Eligible infants were investigated for possible IgE mediated or non-IgE mediated CMA. Results-43 infants were included, mean age 5.7 months. 33/43 (77%) FTT presented a CMA related disease: 3/43 (7%) were diagnosed as presenting an IgE mediated CMA, 30 (93%) had a non IgE-mediated CMA, confirmed by the elimination diet for diagnostic purposes, that led to a significant improvement of symptoms and recrudescence after milk reintroduction. A total of 29 out of 30 patients (one patient was lost at follow-up) moved up to their original growth percentile after dietary changes. Growth z-scores were computed based on WHO anthropometric data. In 10 out of 43 patients (23%) were diagnosed with gastro-esophageal reflux disease (GERD). Conclusions-when evaluating an infant with FTT, physicians should include in their evaluation an extensive search for IgE mediated and non IgE mediated CMA. When in vivo and in vitro analysis are not conclusive, a 4- to 8-weeks trial of CMFD and a consecutive re-introduction of milk proteins may be helpful in less common diagnoses.