One study of dieting obese patients followed them for varying lengths of time. Among those who were followed for fewer than two years, 23 percent gained back more weight than they had lost, while of those who were followed for at least two years, 83 percent gained back more weight than they had lost.  UCLA researchers report in the April issue of American Psychologist, the journal of the American Psychological Association, that 50 percent of dieters weighed more than 11 pounds over their starting weight five years after a diet.