There is considerable evidence for cognitive dysfunction in schizophrenia and affective disorders, but the pattern of potential similarities or differences between diagnostic groups remains uncertain. The objective of this study was to conduct a quantitative review of studies on cognitive performance in schizophrenia and affective disorders. Relevant articles were identified through literature search in major databases for the period between January 1980 and December 2005. Meta-analytic treatment of the original studies revealed widespread cognitive deficits in patients with schizophrenia and affective disorders in intellectual ability and speed of information processing, in encoding and retrieval, rule discovery and in response generation and response inhibition. Differences between diagnostic groups were quantitative rather than qualitative.