AimsTo evaluate an 18-month text-messaging intervention in teenagers with Type 1 diabetes and to assess factors associated with text responsiveness and glycaemic benefit.Methods Teenagers with diabetes (N = 147), aged 13-17 years, received two-way text reminders at self-selected times to check blood glucose levels and reply with blood glucose results.Results At baseline, the participants (48% boys, 78% white, 63% pump-treated) had a mean AE sd age of 14.9 AE 1.3 years, diabetes duration of 7.1 AE 3.9 years and HbA 1c concentration of 69 AE 12 mmol/mol (8.5 AE 1.1%). The mean proportion of days with ≥1 blood glucose response declined over time (0-6 months, 60 AE 26% of days, 7-12 months, 53 AE 31% of days, 13-18 months, 43 AE 33% of days). Over 18 months, 49% responded with ≥1 blood glucose result on ≥50% of days (high responders). Regression analysis controlling for baseline HbA 1c revealed no significant change in HbA 1c from baseline to 18 months in high responders (P = 0.54) compared with a significant HbA 1c increase in low responders (+0.3%, P = 0.01). In participants with baseline HbA 1c ≥64 mmol/mol (≥8%), high responders were 2.5 times more likely than low responders to have a clinically significant [≥5.5 mmol/mol (≥0.5%)] HbA 1c decrease over 18 months (P < 0.05). In participants with baseline HbA 1c <64 mmol/mol(<8%), high responders were 5.7 times more likely than low responders to have an 18-month HbA 1c <58 mmol/mol (<7.5%; P < 0.05).Conclusions Teenagers with Type 1 diabetes who responded to text reminders on ≥50% of days over 18 months experienced clinically significant glycaemic benefit. There remains a need to tailor interventions to maintain teenager engagement and optimize improvements.