Physicians require a screening instrument to detect psychiatric disorders in patients with chronic fatigue syndrome (CFS). Different threshold scores on the Hospital Anxiety and Depression scale (HAD) and the mental health scale of the Medical Outcome Survey (MOS) were compared with two gold standards for the presence or absence of psychiatric disorder, standard diagnostic criteria (DSM-III-R) and a threshold score for the number of psychiatric symptoms at a standardized psychiatric interview (Revised Clinical Interview Schedule total cut-off score of 11/12). They were compared by use of validating coefficients and receiver operating characteristics in 136 consecutive CFS medical outpatients. The HAD scale at cut-off of 9/10 was a valid and efficient screening instrument for anxiety and depression by comparison with both gold standards. The MOS mental health scale at its recommended cut-off score of 67/68 yielded too many false-positives to be recommended as a psychiatric screening instrument in CFS patients.