One of the most serious problems faced by researchers studying eating disorders is denial of illness in individuals with anorexia nervosa. Importantly, the term "denial" not only has different meanings, but in the case of anorexia nervosa its very nature still remains obscure. It is not even known whether it is deliberate or unintentional. Denial of illness in anorexic patients has serious consequences for evaluation of the reliability of information obtained from those individuals. Indeed, the most frequently used screening questionnaires, such as the Eating Attitudes Test (EAT) (Garner & Garfinkel 1979) and the Eating Disorder Inventory (EDI) (Garner et al. 1983), may not reflect the psychological state of the subjects due to distorted responses. The objective of this review article is to elucidate, at least in part, the nature of denial of illness in anorexic individuals and, importantly, to present methods for direct or indirect measurement of this variable. The authors emphasize the detrimental effect of denial of illness on the quality of information obtained from the patients and the notorious unreliability of self-report data. The final part of the paper contains suggestions as to methods of bypassing the pitfalls associated with the influence of denial of illness on the results of studies involving anorexic individuals; for instance, it is recommended that one should build an honest and trustful relationship with the patient. Last but not least, the focus is placed on the potential of experimental psychology, which offers tools producing robust data, resistant to deliberate distortion by patients.