Background Due to changes in dietary habits tree nuts (TN) are consumed in many households and TN allergy appears to be increasing. One risk factor seems to be allergies to other food such as peanuts. The aim of our study was to investigate, how often peanut-sensitized infants and toddlers are co-sensitized to cashew, hazelnut and walnut and to determine the likelihood of its clinical relevance by their 2S albumin-specific (s)IgE. Methods Sera of 101 peanut-sensitized children, 5 to 24 months of age (median 16 months) were analyzed regarding sIgE to hazelnut, walnut and cashew and to their 2S-albumins Cor a 14, Jug r 1 and Ana o 3 as well as to Ara h 1 and 2, by using the NOVEOS TM immunoanalyzer system. Results 96% of the peanut-sensitized children were co-sensitized to at least one TN with 94.1% to hazelnut, 87.1% to walnut and 84.2% to cashew. More than half (58.4%) of the children were sensitized to at least one 2S albumin with similar rates for infants and toddlers, 26.7% to all three. Moreover, sensitization rates were similar in peanut allergic and tolerant children. Estimating the likelihood of clinical relevance, 15.8% of all peanut-sensitized children had an at least 90% probability to be hazelnut and/or cashew allergic. Conclusion TN sensitization seems to be common among peanut-sensitized infants and toddlers. Many had a high likelihood to be TN allergic. Therefore, it should be considered to determine TN-sIgE in peanut-sensitized children if TN are not consumed so far.