Background. Primary arachnoid cysts are benign developmental lesions of arachnoid mater. Arachnoid cysts may be detected due to various neurological symptoms, or they may be encountered as incidental findings of neuroimaging. Consequently, a significant share of the patients seems asymptomatic. There are diverging opinions about the clinical importance of cyst sizes, cyst location and degree of volume reduction after surgery, hence contributing to controversies regarding indications for surgical treatment. We present the first study assessing internationally established parameters of quality of life and mental health in a clinical-outcome analysis of adult patients with arachnoid cysts. Method. Ninety-two adult patients with arachnoid cysts who had been referred to our department over the last 16 years were included. Forty-seven patients had undergone surgery and 45 patients had not been operated on. Data for analysis was based on both medical records and questionnaires sent out by mail. Quality of life was assessed by the Short Form 36 Health Survey (SF-36), and mental health was further evaluated by the Hospital Anxiety and Depression Scale (HADS). Seventy-one percent of patients responded to our questionnaires. Findings. There was a great variation in the presenting symptoms, seemingly without any relation to cyst localisation. Patients with arachnoid cysts seem to have a reduced quality of life and a very high prevalence of anxiety compared to a healthy normal population. Men presented lower outcome scores than women. Subjects with symptoms, that we retrospectively labeled biologically comprehensible, tended to have higher quality of life, less anxiety and better subjective symptom relief after surgery. Conclusion. Our arachnoid cyst population had a low employment status, decreased quality of life scores and prevalent symptoms of anxiety. We argue that the arachnoid cysts are, in most cases, not directly related to these studied parameters. We speculate that our findings may reflect the demographic characteristics of adults likely of being diagnosed with incidental cysts. A better clinical outcome for patients with biologically plausible symptoms supports a neurobiological approach in the selection of patients suited for surgery. © 2007 Springer-Verlag.