A continuous decline in immune responses to Newcastle disease (ND) vaccine was observed in commercial broiler flocks in Croatia, Slovenia, and Bosnia and Herzegovina beginning in 1982. Floating mean haemagglutination inhibition (HI) titres declined from log(2) 4 in 1983 to a low of log(2) 2.4 in 1986, then were log(2) 2.9 in 1990. Several causes of the decline were discounted, leaving mycotoxins in feed and infection with chicken anaemia virus (CAV) as the two most likely causes. Mycotoxins in feed could not be evaluated retrospectively, but archival tissues were available from Croatia and Slovenia. Tissue sections were examined by in situ hybridization for CAV. Whereas only one chicken from early in the decade was infected, all but one of the chickens from late in the decade were. The increase in CAV detection correlated inversely with ND HI titres. Whereas this correlation does not establish cause and effect, CAV cannot be eliminated as a contributory cause of immune suppression.