AIMS/HYPOTHESIS: The aim of this study was to investigate whether the use of antimicrobials is associated with the risk of childhood type 1 diabetes. MATERIALS AND METHODS: The study population included all children born in Finland between 1996 and 2000 who were diagnosed with type 1 diabetes by the end of 2002. For each case (n=437), four matched controls were selected. Data on diabetes and the maternal use of antimicrobials was derived from nationwide registries. RESULTS: Maternal use of phenoxymethyl penicillins (odds ratio [OR]=1.70, 95% CI 1.08-2.68, p=0.022) or quinolone antimicrobials (OR=2.43, 95% CI 1.16-5.10, p=0.019) before pregnancy was associated with an increased risk of type 1 diabetes in the child, whereas the use of other specific antimicrobials was not related to the risk. The risk was also higher among mother-child pairs where macrolides were used both by the mother before pregnancy and by the child, compared with pairs where neither used macrolides (OR=1.76, 95% CI 1.05-2.94, p=0.032). Maternal use of antimicrobials during pregnancy was not associated with an increased risk. The high use of antimicrobials by the child (more than seven vs seven or less purchases) was related to greater risk (OR=1.66, 95% CI 1.24-2.24, p=0.001). CONCLUSIONS/INTERPRETATION: Overall, the use of antimicrobials before pregnancy, during pregnancy or during childhood was not related to the risk of childhood type 1 diabetes. However, the use of some specific antimicrobials by the mother before pregnancy and by the child may be associated with an increased risk. Further studies are needed to confirm these associations and to elucidate the underlying mechanisms of action.