Abstract Objectives The study aim was to identify key factors associated with the health-related quality of life (HRQOL) of patients with type 1 diabetes mellitus (T1DM) treated with neutral protamine Hagedorn (NPH) insulin or human insulin analog glargine (IGLA). Methods We conducted two cross-sectional studies in Minas Gerais State, Brazil. One with 401 patients treated with IGLA, and the other with 179 T1DM patients treated with NPH. HRQOL was measured by Euroqol (EQ-5D-3L). Key findings Most participants were male (51%), aged between 18 and 40 years (47%), non-black (58%) and from the highest economic strata (A1-B2) (74%). Participants perceived their health as good/very good (51%), had one to three medical consultations in the previous year (51%), were not hospitalized in the previous year (74%), did not report angina (96%), diabetic neuropathy (90%), hearing loss (94%) or kidney disease (89%). Non-severe hypoglycaemia episodes in the last 30 days were reported by 17% of participants. Conclusions Higher HRQOL was associated with younger age (18–40 years), good/very good health self-perception, having had up to three medical consultations in the last year, not being hospitalized in the last year, having none to three comorbidities, not reporting angina, diabetic neuropathy, hearing loss or kidney disease and having had episodes of non-severe hypoglycaemia. In addition, the findings of our study demonstrated inequalities in access to treatment, which will be the subject of future research projects.