As more people travel to high altitudes for recreation or work, more travelers with underlying medical conditions will need advice before traveling or treatment for altitude illness. This article focuses on the two main issues for travelers: whether travel to a high altitude will have a negative effect on their underlying medical condition and whether the medical condition increases the patient's risk of developing altitude illness. Although patients with severe pulmonary or cardiac conditions are most at risk in the hypoxic environment, other conditions such as diabetes and pregnancy warrant attention as well.