OBJECTIVE: To investigate the prevalence of adverse reactions to food and food allergy in Icelandic and Swedish 18-month-old children. DESIGN: Prospective multicentre comparative study. SETTING: Primary health care centres in Sweden and Iceland. SUBJECTS: A total of 324 children in Iceland and 328 in Sweden who attended for regular 18-month check-up. MAIN OUTCOME MEASURES: Adverse reaction to food according to questionnaire, and food allergy according to skin prick tests and double blind food challenge tests. RESULTS: Adverse reactions to food were reported in 27% of children in Iceland and 28% in Sweden. Food allergy was confirmed in 2.0% in both countries. Allergy among other family members was reported in 45% of the Icelandic children and 62% in the Swedish (p