Natural and complex disasters can cause a dramatic increase in the demand for emergency medical care. Local health services can be overwhelmed, and damage to clinics and hospitals can render them useless. Many countries maintain mobile field hospitals for defense or humanitarian purposes. Dispatching these facilities to disaster-affected countries would seem an ideal response to emergency medical needs. Unfortunately, experience has shown that in the case of natural disasters, field hospitals often have not met the expectations of recipients and donor institutions. In July 2003, the World Health Organization and Pan American Health Organization sponsored a workshop in El Salvador to discuss the pros and cons of using foreign field hospitals in the aftermath of natural disasters. These guidelines are the result of that workshop. The workshop participants identified different phases when foreign field hospitals and specialized medical personnel are most useful. They can provide advanced trauma care and life support if at the disaster site within 48 hours of the impact of an event; they would provide follow-up care for trauma victims and resumption of routine medical care in the two weeks following the event; during rehabilitation and reconstruction phases (from two months to two or more years), a field hospital might serve as a temporary replacement for damaged health facilities. These guidelines propose conditions that field hospitals and their staff should meet for each of these phases. The guidelines also outline issues that authorities in donor countries and disaster-affected countries should discuss before mobilizing a field hospital.