The aims of this study were to define the development of sensitization and clinical allergy to multiple nut types by age, and to identify associations. This observational cross- sectional study was carried out in a tertiary level allergy clinic. Seven hundred and eighty four nut-allergic children, prospectively enrolled on first attendance with history of a type-1 allergic reaction shortly after definite nut ingestion with evidence of sensitization (presence of nut-specific IgE) by skin prick test (SPT) to peanut, Brazil, almond, hazel and walnut (positive > or = 3 mm). Details of exposure (tolerance or allergy) to each nut were obtained. The main outcome measures were mono or multi-sensitization (specific IgE to one or more than one nut type, demonstrated by SPT); mono or multi-allergy (clinical allergy to one or more than one nut type). By 2 yr of age at least 19% were multi-sensitized, and 2% multi-allergic. Increasing proportions were exposed to multiple nut types with increasing age (23% at 2 yr to 73% by 10 yr) and greater proportions were multi-sensitized (19% at 2 yr to 86% at 5-14 yr) and multi-allergic (2% at 2 yr to 47% at 14 yr). This study is the first to define the natural history of multiple nut allergies in childhood. New findings are that a large proportion of those aged 0-1 yr with nut allergy are already sensitized (have specific IgE) to multiple nut types, implying in utero or early life sensitization; those who present later in childhood are increasingly likely to be sensitized and clinically allergic to multiple nuts. This is related to increased duration of allergy and exposure to multiple nut types with age. Children with nut allergy should avoid all nut types from the onset.