These guidelines on malaria prevention are designed to aid health care workers who advise travellers, particularly those who will be overseas for less than a year. The present, and any future, revisions are the responsibility of the Advisory Committee on Malaria Prevention in UK travellers (ACMP; membership given at the end of the Guidelines). This has replaced the consensus meetings which produced earlier versions from 1980 to the the 1997 version. The guidelines are in three parts. The first part is a summary that emphasises modifications to the advice given in the last set of guidelines, published in 1997. The second part discusses the issues addressed in formulating the guidelines. Oversimplified lists of recommendations by country can be misleading. The second part also addresses the health care worker's consultation with prospective travellers. Doctors, practice nurses and pharmacists are asked to read this section to ensure that due attention is paid to the traveller's history and destination. The third part gives specific recommendations for travellers to specific destinations and some details of individual drugs. Fuller information on some drugs now less used was given in earlier versions of the guidelines. These guidelines reflect experienced professional opinion. Data are inadequate for unequivocal views to be given on several issues, but all available evidence has been taken into consideration. There is often a range of acceptable options, but to meet the requests of general practitioners the guidelines aim to give one recommended option and state the alternatives, suggesting when and how different regimens can be used to good effect. However, there are now several options for effective prophylaxis of highly chloroquine-resistant falciparum malaria, and the choice between them will depend on details of the journey and individual preferences. Decisions on the terms under which different drugs are licensed for use are the responsibility of the Licensing Authority, advised by the Committee on Safety of Medicines and not of the ACMP. The guidelines should therefore be read as a supplement to and not as a substitute for the relevant data sheets. In concept and practice, chemoprophylaxis lies somewhere between vaccinations (for which people expect governments to lay down schedules) and treatment of ill people (which is determined by individual clinical need and choice). The risks of malaria need to be balanced against the risks of the preventive measures, on the basis of the data available. Travellers may ask for an explanation of these risks and doctors and practice nurses need to be well informed and able to present this information to travellers. The second part of these guidelines may also be of use to prospective travellers who wish to read about the options themselves. All readers are recommended to read part two in its entirety to get a balanced picture.