OBJECTIVES: To determine the prevalence of soy allergy in IgE-associated cow's milk allergy (CMA). STUDY DESIGN: Children <3.5 years with documented IgE-associated CMA (n = 93) were evaluated for soy allergy by double-blind, placebo-controlled food challenge, open challenge, or convincing previous history of an anaphylactic reaction to soy. Children tolerant to soy at entry received soy formula and were followed up for 1 year. RESULTS: Of this IgE-associated CMA cohort (ages 3 to 41 months), 14% (95% CI = 7. 7%-22.7%) were determined to have soy allergy, 12 definitely at entry and 1 possibly after 1 year of soy ingestion. The latter child experienced severe failure to thrive at enrollment and exhibited improved growth while receiving soy during follow-up but was diagnosed with eosinophilic esophagitis at study completion. Improved growth (P <.05) occurred in the non-soy-allergic cohort ingesting soy formula (579 31 mL/d) during the year of follow-up. CONCLUSIONS: Soy allergy occurs in only a small minority of young children with IgE-associated CMA. As such, soy formula may provide a safe and growth-promoting alternative for the majority of children with IgE-associated CMA shown to be soy tolerant at the time of introduction of soy formula.