The authors investigated the protein expression of double-stranded RNA-activated protein kinase (PKR), which was identified by using a previous cDNA microarray study, to discover PKR's correlations with several pathological parameters and to elucidate its role in neoplastic transformation and progression of lung adenocarcinomas. Immunohistochemistry for PKR was performed and a semiquantitative scoring method was calculated based on staining intensity and percentage of immunoreactive tumor cells (high vs low) for one bronchioloalveolar carcinoma (BAC), 16 adenocarcinomas consisting of BAC and invasive carcinoma (mixed) and 21 invasive adenocarcinomas without BAC (invasive). The BAC had high-grade expression and the mixed type tended to more frequently show high-grade expression than the invasive type (P = 0.028). There were no significant associations with age, tumor size, lymph node metastasis, lymphovascular invasion or the pathological stage. The Kaplan-Meier survival curves demonstrated that the patients with high-grade PKR expression had significantly shorter survival periods than those patients with low-grade PKR expression (P = 0.018). These results do not support the concept of PKR as a tumor suppressor in small-size peripheral adenocarcinomas of the lung.