Non-small cell lung cancer (NSCLC) is a main group of lung malignancies. Epigenetic changes are as important as genome structural changes in carcinogenesis. MicroRNA (miRNA) is a class of non-coding single-stranded RNAs that play an important role in the regulation of matrix RNA (mRNA) translation and degradation. MicroRNA expression changes occur in many cancers. According to the field cancerization theory, tumor-adjacent histologically normal tissue takes part in tumor progression by triggering cell transformation. The important clinical implication is that the fields may serve as the basis for a recurrence after surgery. Thus, the aim of our study was to determine the expression levels of miRNAs let-7a, miR-155, and miR-205 in tumor and tumor-adjacent apparently normal tissues to evaluate these changes as potential prognostic markers in NSCLC patients. Methods. The expression of miRNAs let-7a, miR-155, and miR-205 in tumor and tumor-adjacent apparently normal tissues at 2 and 5 cm was determined by real-time PCR with subsequent quantification using a 2–ΔΔСt method. The findings were then analyzed to reveal possible associations with clinical and morphological parameters, such as age, cancer stage, and tumor grade. Results. The expression of miRNA let-7a was found to be significantly lower in tumor than that in tumor-adjacent apparently normal tissue at 2 and 5 cm. In groups of patients older than 63 years with Stage III—IV NSCLC, the expressions of microRNA let-7a and miR-155 in tumor tissue were substantially lower than that in the adjacent normal tissue. Beyond that point, patients with high-grade tumors had also a significantly lower expression of miRNA let-7a in relatively adjacent apparently normal tissue. Conclusion. The findings suggest that miRNA let-7a and miR-155 may be used as poor prognostic markers for patients with NSCLC.