We report alterations in leucocytes numbers and lymphocyte subset percentages, determined by flow cytometry, for three observed and previously defined clinical stages (asymptomatic carrier (AC), AIDS-related complex (ARC) and AIDS) of naturally occurring FIV infection. Unstaged FIV-positive cats had significantly lower numbers of total leucocytes (WCC) and neutrophils, lower percentages of PanT+ and CD4+, lower CD4:CD8 ratio and a higher percentage of B cells compared with unstaged FIV-negative cats. When FIV-positive cats were separated into clinical stages and compared with matched FIV-negative cats, AC FIV-positive cats had a significantly lower WCC, lower absolute numbers of neutrophils and lymphocytes, lower percentages of CD4+ and CD8+ cells, lower CD4:CD8 ratios and a higher percentage of B cells than healthy FIV-negative cats. ARC FIV-positive cats had lower percentages of PanT+ and CD4+ cells, lower CD4:CD8 ratios and higher percentages of B cells than matched FIV-negative cats. FIV-positive cats with AIDS had significantly lower percentages of CD4+ cells than matched FIV-negative cats. Comparisons among the three observed clinical stages of FIV-positive cats showed that AC FIV-positive cats had significantly lower WCC, significantly lower absolute numbers of neutrophils and a significantly lower percentage of CD8+ cells than ARC FIV-positive cats. AIDS FIV-positive cats had a significantly lower percentage of B cells than AC or ARC FIV-positive cats. FIV-positive cats had a similar leucocyte response to illness as FIV-negative cats but had consistently lower percentages of CD4+ lymphocytes. Thus, in the staging of FIV, a rise in the percentage of CD8+ lymphocytes could be used to distinguish between AC and ARC and a fall in the percentage of B lymphocytes could distinguish AIDS from AC and ARC.