Background: People with schizophrenia and bipolar disorder have been shown to have a wide range of cognitive deficits, but the longitudinal course of these deficits remains unclear. Some studies have suggested that cognitive function deteriorates over time, whereas others have reported stability or even an improvement in some functions. The objective of the present study was to examine the longitudinal course of cognitive function in a cohort of patients with schizophrenia and bipolar disorder compared with a control group. Methods: Forty-three patients with bipolar disorder, 50 patients with schizophrenia and 24 healthy controls were assessed twice with a neurocognitive battery (Executive Functions, Working Memory, Verbal Memory, Visual Memory, Vigilance and Motor Speed tasks), clinical scales (the Positive and Negative Symptom Scale, the Hamilton Rating Scale for Depression and the Young Mania Rating Scale) and functional outcome measures (WHO's Disability Assessment Scale and occupational adaptation level, and Quality of Live Scale) over a five-year follow-up period. Results: Repeated measures analysis demonstrated no differences between patients and controls in degree of change in neuropsychological performance over this time period: Executive Functions; F=1.27; p=0.280; Working Memory F=0.68; p=0.511; verbal fluency: F=0.033; p=0.718; Verbal Memory, learning: F=1.30; p=0.278; V Verbal Memory, recall: F=1.30; p=0.278; Visual Memory: F=0.117; p=0.842; Vigilance: F=2.71; p=0.103). Discussion: We conclude that in both group of patients, with schizophrenia and bipolar disorder, cognitive impairment remains stable. Overall, the findings are more consistent with a neurodevelopmental model of schizophrenia than with a neurodegenerative model.