OBJECTIVE: To evaluate the effectiveness of a psychological, family-based intervention to improve diabetes-related outcomes in patients with poorly controlled type 2 diabetes. METHODS: This study was a randomized controlled trial of a psychological family-based intervention targeted at individuals with poorly controlled type 2 diabetes. Recruitment and follow-up occurred at specialist diabetes clinics. Patients were randomly allocated to an intervention group (n=60) or a control group (n=61). Poor control was defined as at least 2 of the patient's last 3 glycated hemoglobin (A1C) readings at >8.0%. The intervention consisted of 2 sessions delivered by a health psychologist to the patient and a family member in the patient's home, with a third session involving a 15-minute follow-up telephone call. RESULTS: At 6-month follow-up, the intervention group reported significantly lower mean A1C levels than the control group (8.4% [SD=0.99%] vs 8.8% [SD=1.36%]; P=.04). The intervention was most effective in those with the poorest control at baseline (A1C>9.5%) (intervention 8.7% [SD=1.16%, n=15] vs control 9.9% [SD=1.31%, n=15]; P=.01). The intervention group also reported statistically significant improvements in beliefs about diabetes, psychological well-being, diet, exercise, and family support. CONCLUSIONS: After participating in a family-based intervention targeting negative and/or inaccurate illness perceptions, patients with poorly controlled type 2 diabetes showed improvements in A1C levels and other outcomes. Our results suggest that adding a psychological, family-based component to usual diabetes care may help improve diabetes management.