There have been a number of approaches to defining chronic fatigue syndrome (CFS). Three sets of diagnostic criteria currently exist: the British definition (Sharpe et al., 1989), the Australian definition (Lloyd et al., 1990), and the current US definition, developed in collaboration with the CDC (Fukuda et al., 1994). Within the US, the diagnostic criteria have undergone considerable revision and continue to undergo revision. The first US case criteria for CFS were developed in collaboration with the CDC in 1988 (Holmes et al., 1988). A central problem with CFS case criteria in general is that they have been developed based on expert consensus. Current definitions have not been supported empirically.