Enterovirus infections may initiate and accelerate the beta-cell damaging process leading to Type 1 (insulin-dependent) diabetes mellitus (Type 1 DM). Recent prospective studies have suggested that this can happen long before overt disease and even in utero. We describe an infant, followed regularly from birth, who progressed to clinical Type 1 DM at the age of 14 months. He had a strong enterovirus exposure exceptionally early in life; the first enterovirus infection occurred before the age of 3 months and the second between the age of 9 and 12 months. The first infection probably occurred at birth, when the child had symptoms of a respiratory infection. This infection was followed by the appearance of beta-cell autoimmunity, and clinical Type 1 DM was diagnosed shortly after the second infection. The child had a low level of maternal enterovirus antibodies and short duration of breast-feeding, both associated with increased risk for enterovirus infections during the fetal period and infancy. This case fits with the current hypothesis that enterovirus infections can induce the process resulting in Type 1 DM, especially when occurring early in life. Furthermore, this demonstrates the feasibility of the present study design, which is applicable also in large-scale birth-cohort studies.