The diagnostic sensitivities of the polymerase chain reaction (PCR) and culture were compared and correlated with clinical signs in 5 vaccinated cats and 3 unvaccinated cats that were experimentally infected with feline herpesvirus 1. Conjunctival swabs were taken each day from 0 to 14 days and on 21, 28 and 30 days after challenge. PCR (49.3%) was significantly more sensitive than culture (30.1%) as assessed by an adjusted McNemar's test to account for non-independence of results between days within each cat (P = 0.02). PCR was considerably more sensitive (34.1%) than culture (8.2%) in vaccinated cats (P = 0.001), whereas there was no significant difference in sensitivities in the unvaccinated cats, where the sensitivity of PCR was 74.5% and that of culture was 66.7% (P = 0.17). In vaccinated cats showing clinical signs, the sensitivities of culture and PCR were 14.8% and 55.6% respectively (P = 0.03), whereas in unvaccinated cats the sensitivities were 80.6% and 96.8% respectively (P = 0.07). This study suggests that disease due to feline herpesvirus 1 has been significantly underdiagnosed, particularly in vaccinated cats.