The objective of this study was to compare self-selected Persian Gulf War veterans attending a health assessment program with veterans ascertained in an epidemiological study to determine why Gulf War veterans do, or do not, present for clinical assessment. A postal survey was sent to randomly selected United Kingdom Armed Forces personnel who served in the Persian Gulf conflict. Outcome measures included a symptom checklist, health perception, physical functioning, psychological distress, post-traumatic stress symptoms, and health attributions. A total of 173 survey respondents had also attended the Medical Assessment Program (MAP). MAP attendees were more likely to be female, older, and working part time or not working at all. They had poorer health perception and reported higher levels of illness, and they differed in terms of their health attributions. The belief that one had Gulf War syndrome and attributing health problems to Gulf War service were the most powerful predictors of MAP attendance, even when controlling for the level of physical functioning. The findings suggest that health beliefs rather than symptoms are more important predictors of attendance of an assessment program and that Gulf War veterans who attended the MAP have different characteristics than those who did not. This suggests that MAP patients are unrepresentative of the wider deployment to the Persian Gulf.