Sustained attention deficits measured by the Continuous Performance Test (CPT) have been reportedly proposed as an endophenotype of schizophrenia. One requirement for an endophenotype is that unaffected first-order relatives must show deteriorated performance compared to healthy controls. We investigated 56 schizophrenic patients, 33 nonaffected first-order relatives, and 36 healthy controls in a degraded and an undegraded version of the CPT of the AX type. Performance of relatives and controls was roughly identical whereas schizophrenic patients performed worse right from the beginning. These results add further evidence that a deficit in the CPT performance is not an endophenotype of schizophrenia in accordance with previous studies.