Pick's disease is characterized neuropathologically by distinct tau-immunoreactive intraneuronal inclusions known as Pick bodies and by insoluble tau proteins with predominantly three microtubule-binding repeat tau isoforms. However, recent immunohistochemical studies showed that the antibody specific for exon 10, which encodes the fourth microtubule-binding repeat, detected other tau lesions in Pick's disease. To better define the spectrum of tau pathology in Pick's disease, we used biochemical, immunohistochemical, and ultrastructural techniques to analyze the tau isoform composition in 14 Pick's disease brains. Western blot analysis showed that both three and four microtubule-binding repeat pathological tau isoforms are present in gray and white matter of various brain regions. Using phosphorylation-dependent anti-tau antibodies, we show that major tau phosphoepitopes are present in sarcosyl-insoluble gray and white matter regions of Pick's disease brains. Also, for the first time to our knowledge, we demonstrated that isoforms with four microtubule-binding repeat tau isoforms are present in Pick bodies from selected brains. Isolated tau filaments were straight or twisted and formed by three microtubule-binding repeat or four microtubule-binding repeat tau isoforms. Major tau phosphorylation-dependent and exon 10-specific epitopes were present in filaments. Therefore, Pick's disease is characterized by an accumulations of Pick bodies in the hippocampal region and cortex as well as the presence of three and four microtubule-binding repeat tau pathology in both cortical gray and white matter that distinguish this tauopathy from other neurodegenerative disorders.