I've lost the reference, but the result of a decently-sized study (based on self-reported data) on people who moved from the obese (or morbidly obese) category into the healthy category and stayed there for 12 months:
80% lost the weight with diet and exercise changes; slightly less than 20% lost the weight with diet changes alone; less than 1% lost the weight with exercise alone.
80% lost the weight with diet and exercise changes; slightly less than 20% lost the weight with diet changes alone; less than 1% lost the weight with exercise alone.