Background: Previous studies have shown that qi-gong, a form of mindbody movement therapy, may be beneficial for people with type 2 diabetes; however, no controlled studies have been conducted to examine the predictors and mediators of qi-gong effects on indicators of diabetes control. This study examined the effects of qi-gong on diabetes control and identified the predictors and mediators of these effects. Design: RCT. Setting/participants: The study included forty-one participants (16 men and 25 women; aged 4171 years) with elevated blood glucose levels. Interventions: Participants were randomized to qi-gong intervention or a usual medical care control group. Physical and hematologic measures were assessed at baseline and after 12 weeks. Main outcome measures: The outcomes were indicators of diabetes control (HbA1c, insulin resistance, fasting blood glucose and insulin, and 2-hour blood glucose and insulin) and potential mediators of these (body weight, waist circumference, and leg strength). Data were collected in 2006 and analyzed in 2007 to 2009. Results: Linear regression analyses showed significant between-group differences in favor of the intervention group in weight (p<0.01); waist circumference (p<0.01); leg strength (p<0.01); HbA1c (p<0.05); insulin resistance (p<0.01); and fasting blood insulin (p<0.01) at 12 weeks. Logistic regression analyses showed that the qi-gong intervention was a significant predictor of reduced weight (odds for decreasing by 2 kg=11.14, p<0.01); waist circumference (by 5 cm=22.50, p<0.01); insulin resistance (by 0.2 unit=3.75, p<0.05); and improved leg strength (odds for increasing by 4 stands in 30 seconds=7.00, p<0.01). The effect of the qi-gong intervention on improved insulin resistance was mediated by reduced weight. Conclusions: The qi-gong intervention was associated with improvements in weight, waist circumference, leg strength, and insulin resistance. The mediation analyses highlight the importance of weight reduction in the control of diabetes. Trial registration #: Australian New Zealand Clinical Trials Registry: ACTRN12607000528459.