Determination of serotypes of dengue viruses involved in sequential infections is important since, according to a theory of the pathogenesis of dengue haemorrhagic fever, a particular serotype may be a risk factor. It has been reported in Asia that at least the serotypes involved in the first infections could be serologically identified by the plaque reduction neutralization test (PRNT) because the highest PRNT titres after the second infections corresponded to the serotypes in the first infections. We re-examined the application of this theory of 'original antigenic sin' in Puerto Rico to evaluate its utility in serodiagnosis. Our results showed that it could not be applied reliably because of discrepant results.