Background: Is the increasing prevalence of autistic disorder (AD) a well-documented trend or merely a reflection of the wider recognition of AD among both the public at large and health care professionals? Data from relevant studies are frequently compromised by comparisons of different sites and different diagnostic methods. Objectives: To explore changes over time, we reviewed the following: 1) the frequency of AD diagnoses; 2) the characteristics of the diagnosed children; and 3) the ages of the children when initial concerns were addressed and AD diagnoses made. Method: We compared the case records of children between the ages of 1 and 17 years who were residing in Nordland County, Norway, and who were diagnosed with AD during two different data collection periods: 1992 (Cohort 1) and 2009 (Cohort 2). Results: In Cohort 1, 28 children were diagnosed with AD; 71 children in Cohort 2 received AD diagnoses. The increase was greatest among children with intelligence quotient (IQ) values of at least 70. The proportion of children with genetic syndromes was around 20% in both cohorts. Median age at AD diagnosis did not differ between the two cohorts (4.5 vs. 5.0 years, respectively). When the two cohorts were combined, children with IQ values of 70 or more without a genetic syndrome and those with IQ values of less than 50 with genetic syndromes were diagnosed at approximately the same age (5.5 and 5.3 years, respectively). Both groups were significantly older at diagnosis as compared with children with IQ values of less than 50 without genetic syndromes (3.5 years). Conclusions:The increase in the number of children diagnosed with AD is consistent with findings from international studies. Contrary to predictions, the age at diagnosis was not reduced over time. A higher proportion of children with IQ values in the average range in the latter cohort may have contributed to this. A delayed diagnosis of AD among children with genetic syndromes may indicate that early autism symptoms are attributed to the genetic condition. Clinical implications are discussed.