Previous studies have compared RNA genomes from the different dengue virus serotypes by cDNA-RNA hybridization using dengue-1 virus- and dengue-2 virus-specific cDNA probes. These probes revealed that there is a close genetic relationship between dengue virus serotypes 1 and 4. In this communication, the cDNA-RNA hybridization results using dengue-3- and dengue-4-specific cDNA probes to determine the genetic relatedness of all four dengue virus serotypes are reported. The results indicate that serotypes 1 and 4 are genetically very closely related (sharing about 70% of their genomes as detected by both the dengue-1 and dengue-4 cDNA probes), as are serotypes 3 and 4 (sharing about 50% of their genomes as detected by both the dengue-3 and dengue-4 cDNA probes). Serotype 2 does not seem to be very closely related to the other dengue virus serotypes by cDNA-RNA hybridization analysis.