The 1994 Fukuda case definition (
1), see Appendix A, is often used, but was designed for research purposes in adults and can exclude some young patients with ME/CFS and include others, who are later found to have another illness (
27). The 2003 Canadian clinical case definition (CCC) (
24) is widely used in adults, because it emphasizes the core symptoms of the illness. A pediatric case definition based on the CCC was published in 2006 (
27). The latter is somewhat complex for use in clinical practice.