Poliovirus isolates can be identified according to their genotypes with use of the technique of oligonucleotide fingerprinting. Fingerprint analysis is performed by cleaving the viral RNA genome with ribonuclease T1 and separating the fragments (oligonucleotides) in two dimensions. The larger, structurally unique oligonucleotides distribute into patterns ("fingerprints") highly characteristic of a specific overall RNA sequence. Isolates from the same epidemic have very similar fingerprints. Isolates from distinct epidemics have very different fingerprints, a consequence of the rapid evolution of polioviruses during replication in humans. Similarity in the fingerprints of case isolates provides independent evidence for epidemiologic linkage. Fingerprinting can readily distinguish vaccine-related isolates from wild strains. Contemporary vaccine-related isolates are very probably vaccine-derived because their fingerprints contain characteristic vaccine-strain oligonucleotide spots (types 1 and 3) and because their wild-type parents are unlikely to have survived largely unaltered in the natural environment. Some examples of applications of this technique within different epidemiologic settings are described.