Gaining a diagnosis is considered to legitimate a person's illness, to both the self and the wider social world, while also giving hope that treatments, and possibly a cure, will be found. A further function of diagnosis from the patient's perspective is to give meaning to the illness experience, which is often uncertain and confusing. To do so, a diagnosis must itself have meaning. This paper explores the creation of meaning in a medically unexplained disorder, fibromyalgia syndrome (FMS). Semi-structured interviews, in which the diagnostic process was explored, were conducted with 17 people diagnosed with FMS in the United Kingdom, selected from a hospital database (16 women, 1 man). Documentary analysis was also undertaken on information available from support groups and health professionals. Although initially an acceptable diagnosis to sufferers, FMS was viewed as a mysterious label, which provided no meaning at the time of diagnosis. The sought information was accessed in an attempt to resolve its meaninglessness, but this proved problematic due to the ambiguous definition of FMS within the medical and support group literature, the invisible nature of the illness, and the lack of an environment where these uncertainties could be openly discussed. Informants varied in the degree of longer-term acceptance of a diagnosis of FMS, in relation to the concordance they achieved between the diagnosis and their experience of illness.