Accurate assessment of type 1 diabetes risk in young children requires discrimination between antibodies that are produced by the child and antibodies acquired through the placenta from an islet antibody-positive mother. We studied 682 offspring from mothers with type 1 diabetes and 329 offspring from fathers with type 1 diabetes and nondiabetic mothers for insulin (auto)antibodies, glutamic acid decarboxylase antibodies, and tyrosine phosphatase IA-2 antibodies before age 1 yr and again at age 2 yr to ascertain transience or persistence. Antibodies were detected at age 9 months in 5 (1.5%) offspring from fathers with type 1 diabetes; all were insulin (auto)antibodies only, all persisted and developed multiple antibodies, and 1 developed type 1 diabetes. In contrast, 31 (4.5%) offspring from mothers with type 1 diabetes had antibodies at 9 months; 12 (1.8%) persisted at age 2 yr, and 19 (2.8%) did not persist, suggestive of transient residual maternal antibodies. Multiple antibodies at 9 months were usually persistent (3 of 4 offspring), as were single insulin (auto)antibodies in offspring from mothers with type 1 diabetes (8 of 13 offspring), whereas persistent glutamic acid decarboxylase antibodies (1 of 12) and tyrosine phosphatase IA-2 antibodies (0 of 2) were rare. Offspring with persistent antibodies at age 9 months had a high type 1 diabetes risk (100% by age 5 yr for those with multiple antibodies and 27% for single antibodies at 9 months), whereas offspring with transient antibodies had 0% type 1 diabetes risk (P < 0.01). Transience was associated with very high antibody levels at birth. For insulin (auto)antibodies, the measurement of subclass was also informative. Residual maternal antibody was indicated by similar insulin (auto)antibodies subclasses at 9 months and at birth, whereas different subclasses were indicative of nonmaternal antibody. Moreover, the presence of IgG1-insulin (auto)antibodies was associated with antibody persistence and type 1 diabetes risk. These strategies are helpful in discriminating high and low risk antibodies before age 1 yr and should be important for prognosis and reducing unnecessary parent anxiety.