Schizophrenia and posterior aphasia are easily cross-diagnosed, primarily because of similarities in verbal output, i.e., vagueness, looseness of association, and apparent confusion. Tape-recorded interviews with 8 posterior aphasics and 10 "loose" schizophrenics were transcribed and analyzed to provide guidelines for the clinician to differentiate the two conditions by monitoring verbal expression. Six major differentiating characteristics were identified. The authors present hypotheses that emphasize differing neuroanatomical loci and neuropsychological mechanisms to explain the differences in verbal output in schizophrenia and posterior aphasia.