Objective To evaluate the feasibility of an online interactive treatment environment for adolescents with type 1 diabetes, called Sugarsquare, to supplement usual care. Research design and methods Sugarsquare provides easily accessible contact with the diabetes team, peer support, and treatment overview. Of 120 eligible patients, 62 adolescents aged 11-21 (M = 15.23, SD = 2.00) were assigned to a usual-care group (n = 31) or a usual-care + intervention group (n = 31). Feasibility was assessed in terms of acceptability, demand, practicability, integration, and efficacy in a 9-month study-period. Results Assessment of acceptability and demand revealed that 20 adolescents in the intervention group (65%) logged in at least once; 16 adolescents (52%) logged in repeatedly. Usage resulted in 5795 page-views, 3580 chat-messages, 427 forum-messages, and in 40 private interactions between 11 adolescents (35%) and professionals. Assessment of practicability revealed that all 13 professionals (100%) accessed the intervention. Slow processing speed and security procedures formed obstacles for usage. Assessment of integration showed that international standards for diabetes care (International Diabetes Federation/International Society for Pediatric and Adolescent Diabetes/American Diabetes Association) were met. Assessment of efficacy revealed improvement in the intervention group in evaluation of care (Patients' Evaluation of Quality of Diabetes), F(1,30) = 5.35, p < 0.05, and quality of life, communication ( PedsQL), F(1,30) = 11.65, p <0.05. The latter was correlated with posted chat-messages (r = 0.42, p < 0.05). No between-group differences were found. Conclusions This study shows that Sugarsquare is feasible in adolescents with type 1 diabetes. It meets a demand in adolescents and can support professionals when organizing on-going care according to international standards. Results are promising and next steps are a full-scale randomized controlled trial and subsequent implementation in daily care. [ABSTRACT FROM AUTHOR]