Sammanfattning
Background: Previous reports indicate an increase in incidence of autism spectrum disorders
(ASD). Aims: First, to assess the incidence of diagnosed ASD in children born between 1996
and 1998, based on nationwide inpatient and outpatient register information. Second, to
investigate the incidence rate over time of diagnosed ASD and specifi cally childhood autism,
Asperger ’ s syndrome and pervasive developmental disorder (PDD-NOS) in children born
between 1987 and 1998. Methods: This is population-based cohort study with children born in
Finland between 1 January 1987 and 31 December 2005; a total of more than 1.2 million
children. Children were identifi ed in the Finnish Hospital Discharge Register, and the reported
diagnoses were based on the International Statistical Classifi cation of Diseases (ICD-10, ICD-9).
Results: The annual incidence rate of diagnosed ASD based on inpatient and outpatient register
data was 53.7 per 10,000 (95% CI 50.4 - 57.2). Incidence was 82.6 per 10,000 in boys and 23.6
per 10,000 in girls, yielding a sex ratio (boys:girls) of 3.5:1. We report an eightfold increase in
the incidence rates in children of diagnosed ASD and specifi cally in childhood autism,
Asperger ’ s syndrome and PDD-NOS and born between 1987 and 1992 based on inpatient
register information. Conclusions: Increased awareness of ASD, more precise diagnostic criteria
and changes in practice for diagnosing autism may have had a substantial effect on the
increased incidence of inpatient treated ASD cases from 1987 to 1992. Between 1992 and 1998,
the incidence rate based on inpatient and outpatient service use remained rather stable.