Aim: To assess the relation between islet cell antibody (ICA) positivity and demographic characteristics in an extensive series of first-degree relatives of children with type 1 diabetes (T1D). Methods: Family members of children diagnosed with T1D before the age of 16 years and attending one of 27 participating paediatric units in Finland taking care of children with diabetes were invited to volunteer for an ICA screening program aimed at identifying individuals eligible for inclusion in the European Nicotinamide Diabetes Intervention Trial (ENDIT). The final series comprised 2,522 first-degree relatives (1,107 males) with a mean age of 20.4 (range 0.1–51.9) years, out of whom 390 were fathers, 622 mothers, 717 brothers, and 793 sisters of affected cases. Results: Two hundred and four family members (8.1%) tested positive for ICA with levels ranging from 3 to 564 (median 18) Juvenile Diabetes Foundation (JDF) units. One hundred and five relatives (4.2%) had an ICA level of 18 JDF units or more. Males had detectable ICA more often than females (9.6 vs. 6.9%; p = 0.02). Antibody-positive family members under the age of 20 years had higher ICA levels than the older ones [median 18 (range 3–514) JDF units vs. 10 (range 3–564) JDF units; p = 0.008]. Among the adult relatives (≧20 years of age) antibody-positive females had higher ICA levels than the males [median 10 (range 5–564) JDF units vs. 9 (range 3–130) JDF units; p = 0.04]. Siblings had an increased frequency of high-titre ICA (≧18 JDF units) when compared to the parents (4.8 vs. 3.2%; p = 0.05). Among siblings, we found a higher frequency of ICA positivity in brothers than in sisters (10.8 vs. 6.9%; p = 0.01), and this was also true for high-titre ICA (6.0 vs. 3.8 %; p = 0.04). Geographically, the highest ICA prevalence was seen among relatives living in the middle of Finland (10.4 vs. 7.2% in the other parts of Finland; p = 0.01). Conclusions: These results imply that male gender and young age favour positive ICA reactivity among family members of children with T1D. Siblings test positive for high ICA titres (≧18 JDF units) more frequently than parents. Accordingly, judged from demographic characteristics, the yield of ICA screening in first-degree relatives would be maximized by targeting young brothers of affected cases.