To investigate the effect of flexible intensive insulin therapy (FIIT) and an automated bolus calculator (ABC) in a Danish type 1 diabetes population treated with multiple daily injections. Furthermore, to test the feasibility of teaching FIIT in a 3-h structured course. The BolusCal Study was a 16-week randomized, controlled, open-label, three-arm parallel, clinical study of 51 adults with type 1 diabetes. Patients aged 18-65 years in poor metabolic control (HbAsub 1c 8.0-10.5%) were randomized to the Control (n = 8), CarbCount (n = 21), or CarbCountABC (n = 22) arm. During a 3-h group teaching, the Control arm received FIIT education excluding carbohydrate counting. CarbCount patients were taught FIIT and how to count carbohydrates. CarbCountABC group teaching included FIIT and carbohydrate counting and patients were provided with an ABC. At 16 weeks, the within-group change in HbAsub 1c was -0.1% (95% CI - 1.0 to 0.7%; P = 0.730) in the Control arm, -0.8% (-1.3 to -0.3%; P = 0.002) in the CarbCount arm, and -0.7% (-1.0 to -0.4%; P < 0.0001) in the CarbCountABC arm. The difference in change in HbAsub 1c between CarbCount and CarbCountABC was insignificant. Adjusting for baseline HbAsub 1c in a regression model, the relative change in HbAsub 1c was -0.6% (-1.2 to 0.1%; P = 0.082) in CarbCount and -0.8% (-1.4 to -0. 1%; P = 0.017) in CarbCountABC. Treatment satisfaction measured by the Diabetes Treatment Satisfaction Questionnaire (status version) improved in all study arms, but the improvement was significantly greater in CarbCountABC. FIIT and carbohydrate counting were successfully taught in 3 h and improved metabolic control and treatment satisfaction. Concurrent use of an ABC improved treatment satisfaction further. 23 references