Introduction: Arachnoid cysts are congenital, benign lesions in the brain and are often incidental radiological findings. Frequently, the arachnoid cysts are left untreated; however, recent studies have shown that arachnoid cysts can cause cognitive dysfunction that affect quality of life. Moreover, the function can improve after surgical decompression. Hence, there is controversy regarding symptomatology and treatment effects of arachnoid cysts. The aim of the study was to analyse if arachnoid cysts can cause cognitive impairment and subjective symptoms and if these impairments are reversible after surgical treatment. Material and methods: Twenty-one consecutive patients with radiologically confirmed supratentorial arachnoid cysts were cognitively evaluated using a battery of seven neuropsychological tests. Twelve of these patients underwent surgery and were evaluated before and after surgery. The patients were also evaluated with neuropsychological testing after surgery. Further information was extracted from the medical records. The cognitive test results were compared to standard population values using z-test, and the test results from the surgically treated patients were compared before and after surgery using paired t-test. Results: The surgically treated patients had a statistically significant improvement of neurocognitive test results after surgery in six out of the seven tests (p < 0.05). The total patient group showed lower mean values in all tests when compared to standard population. Statistical significance was, however, only detected in two of the seven tests. All surgically treated patients reported diminished symptoms after surgery. Conclusions: The patients with arachnoid cysts presented with cognitive dysfunction compared to the normal population which improved after surgical decompression. Arachnoid cysts should not be considered asymptomatic unless thoroughly evaluated with clinical and neuropsychological work-up.