The present study aimed at analyzing the possibilities of early diagnostics of attention-deficit/hyperactivity disorder in toddlers and preschool children. Parents and caregivers from children day care centers provided information about 863 children (mean age, 47.18 months; 410 girls and 453 boys). The methods used in the study were as follows: Child Behavior Checklist/1(1/2)-5 (CBCL), Caregiver-Teacher Report Form (C-TRF), and clinical questionnaire for evaluation of attention-deficit/hyperactivity disorder symptoms. The study consisted of two stages: 1) screening of the emotional and behavioral problems of children based on parental and caregiver-teachers' reports; 2) clinical interview with parents of children at risk for attention-deficit/hyperactivity disorder as measured by empirical ratings of attention hyperactivity symptoms. Results revealed that according to parental ratings, attention and hyperactivity problems are related to children's age. According to caregiver-teachers' ratings, boys were rated as having more problems of attention and hyperactivity than girls. Based on the results from the first stage, children at risk for attention-deficit/hyperactivity disorder were analyzed further. Case study analysis showed attention-deficit/hyperactivity disorder symptoms in these children to be a part of overall pattern characterized by behavioral, emotional, and other problems. The quantitative as well as qualitative analysis provides the evidence for a high comorbidity of attention-deficit/hyperactivity disorder and other emotional and behavioral problems in early childhood. Study showed that comprehensive clinical assessment is necessary for early diagnostics of ADHD.