OBJECTIVE To identify environmental factors involved in the etiology of insulin-dependent diabetes mellitus (IDDM). RESEARCH DESIGN AND METHODS An estimated 90% of all incident cases of IDDM in patients 0–14 years of age in New South Wales, Australia, were ascertained over 18 months. For each IDDM patient, two age- and sex-matched control subjects were randomly selected from the population. Past environmental exposures were determined with a questionnaire completed by the parents. Response rates were 92% for the IDDM patients (217 of 235) and 55% for the control subjects (258 of 470). The relative risk associated with each exposure was estimated with the odds ratio (OR) adjusted for confounding factors using multiple logistic regression. RESULTS The introduction of cow's milk-based infant formula into the diet before 3 months of age was associated with an increased risk (OR 1.52, 95% confidence interval [CI] 1.04–2.24). Exclusive breast-feeding for ≥3 months was associated with a protective effect (OR 0.66, 95% CI 0.45–0.97). High dietary intake of cow's milk protein in the 12 months before the onset of diabetic symptoms was also associated with an increased risk (OR 1.84, 95% CI 1.12–3.00). A recent infection (during the 3 months before onset of diabetic symptoms) was more common in the patients than the control subjects (OR 2.92, 95% CI 1.96–4.35), as was day care attendance before the age of 3 (OR 1.73, 95% CI 1.00–3.00). When two age-groups, defined by the median age at onset of diabetes, were compared, the associations with early infant-feeding were stronger among the younger group (CONCLUSIONS These results indicate an increased risk of IDDM associated with early dietary exposure to cow's milk-containing formula, short duration of exclusive breast-feeding, high intake of cow's milk protein in the recent diet, recent infection, and early attendance at day care.