Abnormal tau hyperphosphorylation and deposition in Pick bodies is a major abnormality in Pick's disease (PiD). This is associated with increased expression of the stress-activated protein kinase, p38 kinase, which has the capacity to phosphorylate tau in vitro. The present study has shown increased expression of phosphorylated p38 (p38-P), which does not cross-react with phospho-tau, in sarcosyl-insoluble fractions enriched in abnormal filaments, and hyperphosphorylated tau in the brain of two PiD cases obtained and processed with very short (less than 2 h) post-mortem delay. Immunohistochemical studies have shown p38-P co-localization in 90% of neurons with Pick bodies, whereas no positive cells are encountered in control brains processed in parallel. Moreover, p38-immunoprecipitated from sarcosyl-insoluble fractions in PiD brains is functionally active as it has the capacity to phosphorylate its specific substrate ATF-2. Combined biochemical, immunohistochemical and functional studies indicate that active p38 kinase is expressed in a very high percentage of Pick bodies, thus suggesting a critical role of this kinase in enhancing and perpetuating tau hyperphosphorylation in PiD.