Public health measures are still the most important factor in improving Third World health. However, medicines have an important part to play, and it is now generally agreed that for the very poor populations medicines should be restricted to those on an 'essential drugs list' and should be made available as cheaply as possible. A major problem arises, however, in trying to stop these very cheap supplies 'leaking' back into the private sector, and being sold to the public at high prices. This is particularly true as many countries have a large rich urban elite as well as a much larger poor rural population. It is also important to recognise that research into Third World diseases must be financed by profits on the sale of medicines in advanced countries. A 'cheap drug' policy in rich countries can be very harmful to the long-term health even of the less developed countries.