To assess the reliability of dot-spot hybridization assay for the Potato Spindle Tuber Viroid, more than 1000 potato samples were cross-tested by biological and hybridization methods. Both methods gave similar results with a range of cultivars. The dot-spot hybridization test was reproducible with formaldehyde-denaturated cell-sap samples stored at room temperature for over 6 months. This finding means that samples can be transported from field stations to a diagnostic laboratory and allows for cross-checking of results between laboratories. The data suggest that dot-spot hybridization is suitable for breeding and quarantine purposes.