Aim To examine the effectiveness and safety over a 12-month period of a telemedicine intervention in adults with type 1 diabetes (T1D) treated with insulin pumps. Methods 74 T1D patients on insulin pumps for at least 1 year (mean 19.5 11.5 years) and HbA1 c  ≥ 6.5% (≥ 48 mmol/mol) were randomized to the telemedicine ( n  = 37) or the standard care group ( n  = 37). The intervention group was instructed to download data from insulin pumps and glucometers monthly. They received immediate phone feedback and recommendations for insulin dose adjustment; and face-to-face visits once in 6 months, compared to once every 3 months for the standard care group. Satisfaction with treatment, quality of life and frequency of hypoglycemic events was evaluated. Results The mean changes in HbA1c adjusted to baseline were − 0.08% (0.25 mmol/mol) vs. − 0.01% (0.03 mmol/mol), in the intervention and control groups, respectively ( p  = 0.18) at 12 months, without an increased frequency of hypoglycemia. Patients in the intervention group felt satisfied and interested in continuing with the treatment ( p  = 0.04). The quality of life scores were similar in both groups. Direct total costs were 24% less in the intervention group, and indirect total costs decreased by 22% compared to the year preceding the study. Conclusions Internet-based insulin dose adjustment is as effective and safe as routine care in adults with type 1 diabetes treated by insulin pumps. For suitable patients, some of the time-consuming routine visits may be replaced by user-friendly digital medicine. Clinical trial registration Clinical Trial.gov Identifier NCT01887431.