BACKGROUND: The month of diagnosis in childhood type 1 diabetes shows seasonal variation. OBJECTIVE: We describe the pattern and investigate if year-to-year irregularities are associated with meteorological factors using data from 50 000 children diagnosed under the age of 15 yr in 23 population-based European registries during 1989-2008. METHODS: Tests for seasonal variation in monthly counts aggregated over the 20 yr period were performed. Time series regression was used to investigate if sunshine hour and average temperature data were predictive of the 240 monthly diagnosis counts after taking account of seasonality and long term trends. RESULTS: Significant sinusoidal pattern was evident in all but two small centers with peaks in November to February and relative amplitudes ranging from ± 11 to ± 38% (median ± 17%). However, most centers showed significant departures from a sinusoidal pattern. Pooling results over centers, there was significant seasonal variation in each age-group at diagnosis, with least seasonal variation in those under 5 yr. Boys showed greater seasonal variation than girls, particularly those aged 10-14 yr. There were no differences in seasonal pattern between four 5-yr sub-periods. Departures from the sinusoidal trend in monthly diagnoses in the period were significantly associated with deviations from the norm in average temperature (0.8% reduction in diagnoses per 1 °C excess) but not with sunshine hours. CONCLUSIONS: Seasonality was consistently apparent throughout the period in all age-groups and both sexes, but girls and the under 5 s showed less marked variation. Neither sunshine hour nor average temperature data contributed in any substantial way to explaining departures from the sinusoidal pattern.

Centre of Excellence for Public Health Northern Ireland Queen's University Belfast Belfast UK

Department for Epidemiology and Health Care Research Technical University of Dresden Dresden Germany

Department of Clinical Science University of Umeå Umeå Sweden

Department of Endocrinology and Diabetes University Children's Hospital Podgorica Montenegro

Department of Endocrinology and Diabetology University Children's Hospital Zurich Switzerland

Department of Endocrinology and Genetics University Children's Hospital Skopje Macedonia

Department of Paediatric Diabetes and Endocrinology Centre Hospitalier de Luxembourg Luxembourg

Department of Paediatrics Herlev University Hospital Copenhagen Denmark

Department of Paediatrics Pécs University Pecs Hungary

Department of Paediatrics University Hospital Sestre Milosrdnice Zagreb Croatia

Department of Pediatric and Adolescent Medicine Medical University of Vienna Vienna Austria

Department of Pediatrics 2nd Faculty of Medicine Charles University Prague and University Hospital Motol Prague Czech Republic

Department of Pediatrics Endocrinology and Diabetes Medical University of Silesia Katowice Poland

Department of Pediatrics Ullevål University Hospital Oslo Norway

Department of Pediatrics University Children's Hospital Ljubljana Slovenia

Diabetes Research Center Brussels Free University Vrije Universiteit Brussel Brussels Belgium

Institute for Biometrics and Epidemiology German Diabetes Center Leibniz Institute for Diabetes Research at Heinrich Heine University Düsseldorf Germany

Institute of Endocrinology Lithuanian University of Health Science Kaunas Lithuania

Leeds Institute of Genetics Health and Therapeutics University of Leeds Leeds UK

Nutrition and Metabolic Diseases Clinic N Paulescu Institute of Diabetes and Metabolic Diseases Bucharest Romania

Odense Patient data Exploratory Network University of Southern Denmark Odense Denmark

Pediatric Endocrinology and Diabetes University Children's Hospital Tübingen Germany

Public Health Agency Department of Health Government of Catalonia Barcelona Spain

School of Clinical Sciences University of Bristol Bristol UK

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Trends and cyclical variation in the incidence of childhood type 1 diabetes in 26 European centres in the 25 year period 1989-2013: a multicentre prospective registration study