Recent studies have indicated that thought disorder occurs among manics at least to the same degree as among schizophrenics. The present study assumes that thought disorder can be considered as an abnormality of language whereby listeners are unable to organize speech into a single, coherent "whole." A model of language processing is presented that predicts that the incoherence of manic speech is due to shifts from one coherent discourse structure to another, while the ability of schizophrenic speakers to construct any discourse structure is deficient. A discourse analysis was applied to normal, manic, and schizophrenic speech samples. The two hypotheses were supported. The implications of these findings in light of other investigations of mania and schizophrenia are discussed.