To investigate effectiveness and cost-effectiveness of 6-monthly monitoring compared with 3-monthly monitoring of well-controlled type 2 diabetes patients in primary care. Methods: A pragmatic randomised controlled patient-preference equivalence trial was performed. From April 2009 to August 2010, 2215 patients from 233 general practitioners across the Netherlands were included. Patients were eligible if between 40- and 80-years-old, diagnosed with type 2 diabetes for more than a year, treated by their general practitioner, not on insulin treatment and well-controlled during the last year (HbA1c ≤58mmol/mol, systolic blood pressure ≤145mmHg and total cholesterol ≤5.2mmol/l). Patients without a strong preference for their monitoring frequency were randomised to 3-monthly or 6-monthly monitoring. Follow-up was 18months. The primary outcome is the percentage of patients remaining under: HbA1c ≤58mmol/mol, systolic blood pressure ≤145mmHg and total cholesterol ≤5.2mmol/l. Equivalence was assumed if the two-sided 95% confidence interval (CI) was between -5 and 5%. Cost-effectiveness was determined using a cost-minimisation analysis. Results: In the 3-monthly group 69.5% remained under good cardiometabolic control, versus 69.8% in the 6-monthly group (difference: 0.3%; 95%CI: -6.2-6.7%). All secondary outcomes were equivalent for 3-monthly and 6-monthly monitoring, except the systolic blood pressure target, physical activity and antihypertensive drug use. Six-monthly monitoring was €387 (£333) cheaper per patient compared to 3-monthly monitoring during the study period. Conclusions: Patients with good cardiometabolic control and without preference for their monitoring frequency can visit the primary care physician less often. The cost-savings can be considerable.