Background : During the last years, an increase in the incidence of DM1 in infants has been observed. Objective : to study the number of children younger than 5 years-old diagnosed with DM1 and compare clinical and laboratory features with older children at DM1 onset. Method : Study of the clinical and laboratory characteristics in subjects diagnosed with DM1 from 1998 to 2003 in Santiago. Patients were classified according to age in 3 groups: 0-4 (GI), 5-9 (GII) and 10-14 years-old (GIII). Results : 19,7% cases were younger than 5 years-old (GI n = 27), GII (43,8% n = 60) and GIII (36,5% n = 50). A shorter duration of symptoms was observed in GI (18,4 ± 23,7 vs 26,4 ± 27,4) (p 0,05) and metabolic acidosis was more severe in GI (pH 7,14 ± 0,1 vs 7,19 ± 0,2 in GII and 7,26 ± 0,1 in GIII) (p 0,05). Conclusions : An important percentage of DM1 in children presents in subjects younger than 5 years-old. This group showed acute and severe clinical presentation with longer duration of symptoms, severe acidosis and lower HbA1c levels. It is necessary to evaluate carefully in order to suspect the diagnosis in this group.