To investigate the incidence of schizophrenia in the age group 0 to 18 years from 1971 to 2010, and compare this with the incidence of all psychiatric disorders.

Cases are selected from the Danish Psychiatric Central Research Registry (DPCRR) which includes data from all admissions to psychiatric hospitals in Denmark. The first case group is all diagnosed with an F295 (before 1994) or F20 (after 1994) schizophrenia diagnosis. The second is all incident patients with a psychiatric diagnosis. We calculated incidence rates (IR) for each year 1971–2010.

The mean IR of early onset schizophrenia (EOS) is lower in the period 1971–1993 compared to 1994–2010, for males as well as females. From 1994 to 2010 there is a significant increase in IR. From 1971 to 1993, males have a significantly higher IR than females. From 1994–2010, females have a significantly higher IR than males. Finally, EOS patients are diagnosed at a significantly lower age in the period 1994–2010 compared to 1971–1993, but this difference is not significant when stratified by sex.

There is a general increase in schizophrenia and all psychiatric disorders diagnosed over the four decades that this study spans. There is a change in the sex distribution of EOS between males and females: from an initial male dominance to an even distribution. For all psychiatric diagnosis, male predominance persists throughout the study. The increase cannot be explained by a single factor, e.g. changes in diagnostic system, organizational structure or diagnostic practice.