Among patients with obstructive sleep apnea syndrome (OSAS), impairment of cognitive function, i.e. deficits in memory, attention, and visuconstructive abilities are common. We applied different forms of treatment for patients with newly diagnosed OSAS in a randomized study with a one-year follow-up. Patients with BMI > 40 kg/m2 were excluded. After the initial diagnostic work-up, male patients were considered to be candidates for either nasal continuous airway pressure (nCPAP) (27 patients) or surgical treatment (uvulopalatopharyngoplasty with or without mandibular osteotomy) (23 patients). Within the groups, the patients were then randomized to active treatment (nCPAP/surgery) or to conservative management. Cognitive function and severity of OSAS were assessed prior to treatment and 3 and 12 months later. At 12 months, all patients on nCPAP had a normal ODI4 index (< 10), and were significantly less somnolent than their controls; 3/11 of the surgically treated patients had a normal ODI4 index. Daytime somnolence was significantly less severe in the surgically treated patients than in their controls. Cognitive function did not correlate importantly with daytime sleepiness or severity of OSAS; the best Pearson pairwise correlation coefficient was between ODI4 and the Bourdon-Wiersma (r = 0.36). Success in treatment of OSAS did not affect neuropsychological outcome. We concluded that the standard cognitive test battery is insufficiently sensitive to identify positive changes in patients with OSAS, especially among those with a high level of overall mental functioning.