Background We examined whether motor speed assessed by the finger tapping test predicts generalized and specific stable deficits because of a common patho-genic process in bipolar and schizophrenic patients. Methods: One hundred and two patients underwent a battery of neuropsychological tests. Patients with a score of less than one standard deviation from their siblings' sample in two assessments with an interval of one year were defined as suffering from stable deficits because of a common pathogenic process. In addition to univariate analyses, factor analyses, ordinal logistic regression, and multiple linear regressions were used. A general score was also calculated. Results: No differences were found between schizophrenic and bipolar patients in the deficits of verbal fluency, shift reasoning ability and executive attention. Schizophrenic patients had greater persistent cognitive deficit because of a common pathogenic factor in the verbal memory dimension than bipolar patients. Motor speed predicted the specific deficits of verbal fluency, shift reasoning, executive attention and the general deficit of both bipolar I and schizophrenic patients. Bipolar patients suffered a lesser specific deficit in the verbal memory dimension than schizophrenic patients did, this domain not being predicted by motor speed. Motor speed predicted the generalized deficit and the specific dimensions in which schizophrenic and bipolar patients showed no differences. Conclusions: These results suggest the presence of general and specific stable cognitive deficits because of a common pathogenic factor related to psychomotor slowness. Motor speed seems to be suitable endophenocognitype for schizophrenia and bipolar disorder.