The paper explores, in detail, the process of a single assessment as it unfolds session by session. It draws attention to the inevitability of the clinician's harbouring preconceptions of all kinds — especially when working with adolescents. It focuses on the difficulty of withstanding those preconceptions and allowing the patient's true anxieties, conscious and unconscious, to emerge. The process is a complex one requiring close attention to transference and countertransference manifestations, to the central significance of observation, to Bion's notion of the opacity of memory and desire. The case, that of a hypochondriacal 17-year-old young woman, highlights some main concerns and defences of the age group, for example, somatization, separation, fears of madness, issues of identity and, most fundamental of all, sexuality.