Background: Gold nanoparticles (AuNPs) exhibit a unique phenomenon known as Surface Plasmon Resonance, which is responsible for their intense red color. This color changes to blue upon aggregation of AuNPs. Objective: This work aims to develop a rapid, simple and cheap assay for direct detection of unamplified HCV RNA extracted from clinical samples using unmodified AuNPs. Methods: Serum samples were collected from healthy volunteers (n=45) and chronic HCV patients (n=30). Extracted RNA, hybridization buffer containing PBS, and a primer targeting the 5'UTR of HCV were mixed. The mixture was denatured, annealed, and then cooled to room temperature for 10. min followed by addition of AuNPs. Results: Salt, primer, AuNPs concentrations and annealing temperature and time were all optimized. In HCV positive specimens, the color of the solution changed from red to blue within 1. min. The assay has a sensitivity of 92%, a specificity of 88.9%, and a detection limit of 50 copies/reaction. Conclusions: To our knowledge, this is the first assay that allows the detection of unamplified HCV RNA in clinical specimens using unmodified AuNPs. The developed assay is highly sensitive, has a turnaround time of 30. min, and eliminates the need for thermal cycling and detection instruments. © 2010 The Canadian Society of Clinical Chemists.