We previously demonstrated the activation of caspase-6 (Casp-6) in the hippocampus and cortex in cases of mild, moderate, severe, and very severe Alzheimer disease (AD). To determine whether Casp-6 is also activated in familial AD, we performed an immunohistochemical analysis of active Casp-6 and Tau cleaved by Casp-6 in temporal cortex and hippocampal tissue sections from cases of familial AD. The cases included 5 carrying the amyloid precursor protein K670N and M671L Swedish mutation, 1 carrying the amyloid precursor protein E693G Arctic mutation, 2 each carrying the Presenilin I M146V, F105L, A431E, V261F, and Y115C mutations, and 1 with the Presenilin II N141I mutation. Active Casp-6 immunoreactivity was found in all cases. Caspase-6 immunoreactivity was observed in neuritic plaques or in some cases cotton-wool plaques, and in neuropil threads and neurofibrillary tangles. These results indicate that Casp-6 is activated in familial forms of AD, as previously observed in sporadic forms. Because sporadic and familial AD cases have similar pathological features, these results support a fundamental role of Casp-6 in the pathophysiology of both familial and sporadic AD.