We studied the incidence of type 1 diabetes in the offspring of patients with childhood-and adolescent-onset type 1 diabetes and several risk factors predicting the risk. We defined the diabetes status in the offspring of all probands who were included in the nationwide register of Finnish type 1 diabetic patients diagnosed at the age of <17 years from 1965 T he incidence of childhood type 1 diabetes has increased globally over the past decades (1,2). The risk of type 1 diabetes in the offspring of diabetic parents by the age of 20 years is ϳ4 -5%, depending on the population where the studies have been conducted and differences in the study design (3-7). It has been suggested that the increase in the incidence of type 1 diabetes has been more rapid in the offspring of type 1 diabetic patients than in the background population (3).The sex difference in the recurrence risk of type 1 diabetes has been detected by a number of studies: the offspring of the fathers who were affected with type 1 diabetes have an increased risk for type 1 diabetes compared with the offspring of affected mothers (3,4,6,8). Preferential cross-sex transmission has also been detected by some studies (3,9) but not by others (4,8).Large, population-based studies with optimal study design are sparse where the ascertainment of offspring is through diabetic parents and complete. Our aim was to assess the recurrence risk of type 1 diabetes in the offspring of a Finnish population-based cohort of patients with childhood-and adolescence-onset type 1 diabetes. We also investigated possible sex-related effects in the transmission of type 1 diabetes from the diabetic parents to their offspring. In addition, we were able to study temporal trends in the incidence during 1970 -2003 and characterize several variables that might have influenced the recurrence risk of type 1 diabetes in the offspring.
RESEARCH DESIGN AND METHODSWe defined the diabetes status of all offspring of all probands who are included in the nationwide register of Finnish cases with type 1 diabetes diagnosed before age of 18 years between 1965 and 1979 (n ϭ 5,144). The cohort was originally used in the Diabetes Epidemiology Research International (DERI) mortality study (10,11). Briefly, the register was initially based on the central drug register (CDR) of the Social Insurance Institution, including patients approved to receive free-of-charge medication for certain diseases including diabetes. The case ascertainment in this cohort was virtually complete (12,13). Their offspring and the other parent of the offspring were identified through the national population register by computer linkage using the unique personal identifier that is assigned to all residents of Finland. The offspring were mainly born between 1970 and 2001, only four of them before 1970. The diabetes status of the offspring was ascertained through several sources: from the nationwide Hospital Discharge Register for years 1970 -2003, from the nationwide Finnish Diabetes Register for children and young adults, and f...