In the last several decades, a great number of studies have pointed to a dramatic increase of type 1 diabetes mellitus (T1DM) incidence in the whole world, especially in younger age groups. Therefore, the aim of the study was to assess changes in the age distribution at onset of T1DM in Montenegro children aged < 15 years during a 15-year period (1997-2011) and analyze the seasonal pattern. Primary case ascertainment was from diabetes register, secondary and tertiary independent data sources were hospital case records and register of children receiving free test stripes in pharmacy. Standardized incidence rates were calculated using the Poisson regression. Case ascertainment was 100% complete using the capture-recapture method. The mean age-standardized incidence was 18.6/100,000 (95% CI: 13.0-24.1) from 2007 to 2011 compared with 13.4/100,000 95% CI, 11.5-15.5) from 1997 to 2006. The incidence of T1DM increased predominantly in younger age groups. Relative increase of incidence per 5-year period was largest in boys aged 0-4 and 5-9 years: 64.7% (95% CI: 20.6-10.7; p = 0.004) and 52.8% (95% CI: 16.9-88.8; p = 0.004), respectively. Seasonality in monthly case counts of T1DM was apparent. The greatest number of cases were diagnosed during autumn and winter months. In conclusion, the onset of T1DM was found to occur at an ever younger age in Montenegro children. Our results indicated a seasonal pattern of the disease onset.