A review of case notes before and after the introduction of the DST into clinical psychiatric practice revealed considerable changes in diagnosis and management. Specifically there were increases in the diagnosis of biological depression and treatment with somatic antidepressant therapy. There was no association between DST results and particular management plans. There was a strong association between requesting the DST and management with antidepressants. It is suggested that the introduction of laboratory tests for psychiatric disorders may firm the belief of psychiatrists in the biological basis of some forms of depression and thus alter their diagnostic and treatment practice.