It has been shown in a record study (Welner, Liss, Robins and Richardson, 1972), and confirmed in a follow-up study using a blind structured interview (liss, Welner, and Robins, 1972), that most of those in-patients in our studies who were discharged without a diagnosis met the criteria for a psychiatric diagnosis when rigorous criteria were used (Feighner, Robins, Guze, Woodruff, Winokur and Munoz, 1972). However, using methods and evidence from these studies, out of the 109 patients who were followed-up for a mean period of 39 months, 25 patients were conspicuous in that even after initial or repeated admissions to hospital, after their records were reviewed, and after a follow-up study, they still remained undiagnosable. It was the purpose of this study to describe and evaluate in detail this group of 25 patients with particular reference to the possible reasons why they remained undiagnosed and therefore were defined by us as ‘undiagnosable’.