MicroRNAs are evolutionarily conserved non-coding components of the transcriptome that can post-transcriptionally control gene expression. Altered microRNA expression has been found to be a common feature of several cancers, including lung carcinomas. The biogenesis and maturation of microRNAs is known to be mediated by the ribonucleases Drosha, Dicer and Ago2. The purpose of the present study was to investigate the expression and distribution of Drosha, Dicer and Ago2 in human non-small cell lung carcinomas (NSCLC) and to relate the respective expression patterns to clinocopatholical features. We used five human NSCLC-derived cell lines and primary formalin-fixed paraffin-embedded tissue samples from 83 NSCLC patients. Drosha, Dicer and Ago2 mRNA and protein expression levels, and their sub-cellular distributions, were assessed using RT-PCR, Western blotting, immunofluorescence and immunohistochemistry, respectively. We found that Drosha, Dicer and Ago2 were expressed in all the cell lines and primary neoplastic and non-neoplastic tissue samples tested. The intensity of the immunohistochemical staining was found to be significantly lower in stage I tumors compared to normal lung tissues. Dicer expression was found to be significantly higher in stage II compared to stage I tumors, and in stage III compared to stage II and stage I tumors. Our results point at a role of Drosha, Dicer and Ago2 in the development of NSCLC and suggest that Dicer may be implicated in the progression of these tumors to advanced stages.