P>Objective Accelerated early growth prior to childhood type 1 diabetes onset is associated with an increased risk for type 1 diabetes (T1D). We aimed to study early growth, correcting for the previously neglected confounder of familial effects. Design Infant growth was studied in a retrospective family case-control study of diabetic children in which siblings acted as matched familial controls allowing correction for confounders related to family particulars. Patients Weight and height data were collected from 213 juvenile onset type 1 diabetic children and their 255 healthy siblings. Growth in the first 4 years of life was studied using repeated measurement. The degree of early overgrowth was correlated with age of clinical onset. Results Birth weight and length did not differ between later diabetic children and their siblings. In the first year of life, weight standard deviation score (SDS) differed between patients and sibs (P = 0 center dot 0001). After the first year, both diabetic children and sibs showed parallel enhanced weight and height gain SDS until age 4 years. Earlier onset diabetes was associated with a higher weight SDS at 6 months of age. Conclusion In this family case-control study the association of increased growth with development of T1D is limited to the first year of life implying that increased growth beyond the first year can be attributed to familial growth patterns, rather than predisposition to T1D per se. Age at disease onset correlated with increased weight in the first 6 months of life, indicating importance of features very early in life on later development of T1D.