The random amplified polymorphic DNA (RAPD) technique was used to determine genetic differences among isolates of Trichomonas vaginalis and these genetic data were correlated with patient records. A panel of 10 random primers was used to determine the type and extent of intraspecific polymorphism in 40 isolates of T. vaginalis. All primers detected DNA polymorphism among isolates. Numerical analysis of 124 RAPD amplified bands generated by these 10 primers were carried out with the unweighted pair group methods analysis (UPGMA) using Jaccard's Similarity Coefficient and data were used to construct a dendrogram. Four main groups can be distinguished by RAPD data, these groups coincide with four different patient categories (asymptomatic and symptomatic: light, moderate, and severe infection). These patients did not have any concomitant vaginal infection. Each of the four groups can be characterized by specific genetic markers, but a specific 490bp marker was found to be specific for all symptomatic isolates, not the asymptomatic isolates. This is the first description of a possible virulence marker for T. vaginalis. Further studies will be necessary to ascertain the importance and function of this genetic marker in clinical infection.