Three groups participated in a slimming programme run by lay people. The course lasted eight weeks, and the average weight loss was about 7 kg both in a small well-controlled study of 33 women and in a much larger one comprising over 10,000 people. Both studies were prospective. The long-term effect was studied in a random sample of about 1000 people. After four years 30-35% of the participants had kept the initial weight loss or were still reducing their weight, and only 15% had regained all the lost weight or more. That this result was better than usual may be because the programme operated through small groups of eight to 12 members run by the obese people themselves. The "slim-club hostesses' had all been obese and had succeeded in losing weight in the same programme. Group pressure and competition may also be important. Finally, the programme tried to change life styles, encouraging more exercise and reduced consumption of alcohol. The combination of scientific methods (behaviour therapy) and lay work may provide a clue for solving obesity and other serious health problems.