Objective Psychiatric diagnoses of patients with type 1 diabetes mellitus (T1DM), the severity of attention deficit/hyperactivity disorder (ADHD) symptoms of the patients and their primary caregivers, and the effects of these factors on treatment were investigated. Methods Sixty-one patients with T1DM were included in the study along with their parents. Psychiatric diagnoses of the patients were determined using a semistructured psychiatric interview, and their depression and ADHD symptom severities were evaluated with self-report scales. The ADHD symptom severities of the parents were evaluated using self-report scales. The relationships among the psychiatric symptoms and the hemoglobin A1c (HbA1c), fasting blood glucose (FBG), and postprandial blood glucose (PBG) levels of the patients were investigated. Results HbA1c levels were found to correlate with the hyperactivity levels of children and the number of diagnoses they had. FBG and PBG values of patients diagnosed with ADHD were found to be higher than in those who did not have ADHD. HbA1c, FBG, and PBG values of the patients who had any disruptive behavior disorder were found to be higher than in those who did not. ADHD total scores, gender (being female), having diagnoses of ADHD or depression were found to be predictive of HbA1c levels according to the regression analyses. No relationship between the clinical findings of the children and their parents' ADHD levels was found. Conclusions The findings of this study implicate that children with T1DM should be evaluated in terms of ADHD which could have negative effects on the treatment.