The still unsolved pathogenesis of Alzheimer's disease (AD) has been the subject of extensive speculation. Some years ago, a local acute phase reaction involving production of interleukin-1 (IL-1) and IL-6 was proposed as the triggering event in AD. Since it has been reported that these cytokines induce expression of intercellular adhesion molecule-1 (ICAM-1), we analyzed AD brain tissue cryosections for the presence of ICAM-1 by immunostaining and for ICAM-2 expression as a control. In senile plaques a marked diffuse or granular staining for the ICAM-1 domains 1, 4, and 5 was observed, whereas ICAM-2 expression was observed in microglial cells. Immunoprecipitation analysis demonstrated the presence of a 85 kd ICAM-1 molecule in AD frontal cortex. Our findings indicate that ICAM-1 accumulates in senile plaques as a complete 5-domain molecule at a relatively early stage of senile plaque formation. Our results are in support of a cytokine-mediated pathogenesis of senile plaque formation.