Aim: The aim of this study was to analyse the risk factors for preschool wheeze with special reference to the early introduction of fish and early antibiotic treatment. To avoid reverse causation regarding antibiotics, we focused on the influence of broad-spectrum antibiotics given during the first week of life. Methods: Data were obtained from a prospective, longitudinal study of a cohort of children born in western Sweden where 50% of the birth cohort was randomly selected. The parents answered questionnaires at 6 and 12 months and at 4.5 years of age. The response rate at 4.5 years was 83% (4496 of 5398 questionnaires distributed). Results: In the multivariate analysis, broad-spectrum antibiotics in the first week increased the risk of recurrent wheeze (=3 episodes) during the last 12 months at age 4.5 years (adjusted OR 2.2; 95% CI 1.3-3.8) and multiple-trigger wheeze (aOR, 2.8; 1.3-6.1). The introduction of fish before the age of 9 months reduced the risk of recurrent wheeze (aOR, 0.6; 0.4-0.8). Conclusion: Treatment with broad-spectrum antibiotics during the first week of life increased the risk of recurrent wheeze and multiple-trigger wheeze at preschool age. The early introduction of fish reduced the risk of recurrent wheeze.