Oral involvement in the pediatric age does not occur as an isolated event, frequently involves a systemic disease being Leukemias -especially B-cell Acute Lymphoblastic Leukemia (B-cell ALL)- and Type 1 Diabetes Mellitus (T1DM) the most relevant. Objective: To find which oral manifestations are observed with greater more frequently in pediatric patients with B-cell ALL or T1DM and to relate these findings with their pathophysiology. Methodology: We formed three study groups, with five patients each: 1) children with T1DM, 2) infants with B-cell ALL, and 3) healthy children. The age range was five to fifteen years. Data were collected from: 1) clinical examination, 2) interrogation of parents or guardians, and 3) review of the clinical record, and 4) Blood count (BC).