The clinical and epidemiological features of feline calicivirus associated vaccine reactions and breakdowns were investigated. Twenty per cent of 123 vaccine reactions were associated with acute oral/respiratory disease alone, and 80 per cent were associated with lameness either alone or in association with other clinical signs. Feline calicivirus was isolated from oropharyngeal swabs from 69 per cent of the vaccine reaction cases. Twenty-four of 31 vaccine breakdowns were associated with acute oral/respiratory disease and only seven with lameness; the virus was isolated from 28 of the 31 breakdowns. Some of the viruses isolated from the vaccine reactions were compared with the appropriate vaccine virus in virus neutralisation tests; the majority appeared to be different from the vaccine virus and were presumably field viruses. However, some appeared more similar to the vaccine virus and the majority of these were associated with lameness after the first vaccination.