The effect of a number of host and environmental factors on the onset of type 1 diabetes mellitus (DM1) in a group of Lebanese children and young adults was studied. Results showed that DM1 in a group of 253 patients presented no gender preference and that the age of onset was similar in both genders. The overall body mass index reflected good metabolic control. HbA1c had a mean value of 8.98%, suggesting poor glucose control. Family history of DM1 and type 2 diabetes mellitus as well as consanguinity in patients' families were not different from those reported in the literature. Finally, onset of DM1 showed seasonal variation, peaking during winter months. DM1 showed a higher prevalence of onset among children born first and a decreased incidence as birth order increased. This study provides valuable data for the diagnosis, control and prevention of DM1 in children.