Molecular imaging techniques developed to ‘visualize’ amyloid in vivo represent a major achievement in Alzheimer’s disease (AD) research. This pooled analysis of four studies determined the level of association between uptake of the fibrillar amyloid β positron emission tomography (PET) imaging agent [18F]flutemetamol (Pittsburgh Compound B analog with a 5.5 times longer half-life to enable it to be used in the clinical setting) and neuritic plaques and fibrillar amyloid β measured by pathologic staining of cortical region biopsy samples. Fifty-two patients with suspected normal pressure hydrocephalus underwent prospective (n = 30) or retrospective (n = 22) [18F]flutemetamol PET imaging for detection of cerebral cortical fibrillar amyloid β and cortical brain biopsy during intracranial pressure measurement or ventriculo-peritoneal shunting. [18F]Flutemetamol uptake was quantified using standardized uptake value ratio (SUVR) with cerebellar cortex as the reference region. Tissue fibrillar amyloid β was evaluated using immunohistochemical monoclonal antibody 4G8 and histochemical agents Thioflavin S and Bielschowsky silver stain, and an overall pathology result based on all available immunohistochemical and histochemical results. Biopsy site and contralateral [18F]flutemetamol SUVRs were significantly associated with neuritic plaque burden assessed with Bielschowsky silver stain (r spearman’s = 0.61, p = 0.0001 for both), as was the composite SUVR with biopsy pathology (r spearman’s = 0.74, p < 0.0001). SUVR and immunohistochemical results with 4G8 for detecting fibrillar amyloid β were similar. Blinded image evaluation showed strong agreement between readers (κ = 0.86). Overall sensitivity and specificity by majority read were 93 and 100 %. Noninvasive in vivo [18F]flutemetamol PET imaging demonstrates strong concordance with histopathology for brain fibrillar amyloid β, supporting its promise as a tool to assist physicians with earlier detection of the disease process and making diagnostic decisions about concomitant AD and other diseases associated with brain amyloidosis.