["We assessed the relationships between supportive and obstructive family behaviors and patients' diabetes self-care activities and HbA1C, and potential interaction effects and differences by demographic characteristics.", 'In a cross-sectional study, 192 adults with type 2 diabetes completed the Diabetes Family Behavior Checklist-II, the Summary of Diabetes Self-Care Activities, and a glycemic control (HbA1C) test.', 'Participants reported similar rates of supportive and obstructive behaviors that were positively correlated (rho=0.61, p<0.001). In adjusted analyses, supportive family behaviors were associated with adherence to different self-care behaviors (β=0.20 to 0.50, p<0.05), whereas obstructive family behaviors were associated with less adherence to self-care behaviors (β=-0.28 to -0.39, p<0.01) and worse HbA1C (β=0.18, p<0.05). Supportive behaviors protected against the detrimental effect of obstructive behaviors on HbA1C (interaction β=-0.22, p<0.001). Non-Whites reported more supportive and obstructive behaviors than Whites, but race did not affect the relationships between family behaviors and self-care or HbA1C.'] Is family involvement helpful and harmful to patients ' self-care and glycemic control?