Attention deficit/hyperactivity disorder (ADHD) is often poorly understood, and treatment practices are variable. This 12-month, prospective, observational study provides information about the diagnosis, co-morbidities, treatment patterns, and quality of life (QOL) of patients aged 6-17 years with ADHD symptoms from eastern Asia and central and eastern Europe. Here, we present baseline data for the 1068 enrolled and eligible patients in the study (median age 8 years, 82.2% male). Patients were grouped into two cohorts based on whether they were prescribed psycho- and/or pharmacotherapy (n = 794) or not (n = 274) at study entry. On average, patients receiving treatment were significantly older (9.1 vs. 8.4 years, p < 0.001), more severely ill (Clinical Global Impressions [CGI]-ADHD-S, 4.6 vs. 4.2, p < 0.001; Child Symptom Inventory-4 Parent Checklist (CSI-4) ADHD:C, 35.2 vs. 31.9, p < 0.001), and had significantly higher CSI-4 symptom severity scores relating to various co-morbidities than patients not receiving treatment. At study initiation, patient's health-related QOL was significantly impaired as measured on the Child Health and Illness Profile-Child Edition (CHIP-CE) rating scale, with significantly more impairment in the treated group of patients for the Comfort, Risks Avoidance, and Achievement domains. These results provide a description of ADHD and treatment practices in these regions and establish a baseline for gauging changes over time in the study sample.