Despite the potential of feline immunodeficiency virus (FIV) as an animal model for human immunodeficiency virus (HIV) studies, the long term effects of naturally-occurring infection have not been determined. HIV infection causes an ongoing deterioration in immune function which directly correlates with disease, in particular acquired immunodeficiency syndrome (AIDS). However, it is not known whether FIV-induced immunosuppression is progressive or related to the clinical condition. This study examined changes in lymphocyte subset numbers of serial samples, taken from cohorts of FIV-positive and FIV-negative cats over and 18-month period. FIV-positive cats were clinically staged as asymptomatic carriers (AC) or cats with AIDS-related complex (ARC), and FIV-negative cats matched and staged on the basis of similar diseases. During the course of the study, 4 FIV-positive cats developed AIDS, classed as the terminal stage of infection. There were no significant differences in the mean absolute numbers of any lymphocyte subset between the onset (t0) and the completion (t18) of the study. Similarly there were no significant changes in subset numbers during the 18 months preceding the development of AIDS. While the study period was brief and the sample sizes small, it is postulated that FIV infection in Australia may not necessarily cause progressive immunodeficiency and that FIV-induced immunosuppression (as measured by subset analysis) may not be well correlated with the clinical status of the infected cat.