Twenty-five haemophiliacs who had been infected with HIV in 1982 or 1983 were followed up from 1986 to 1993. The absolute number of the CD4+ and CD8+ cells, neopterin levels and more recently the percentage of activated, DR+ T lymphocytes were determined twice a year. In most patients a permanent decline in the CD4+ cell count was observed whereas in two HIV-infected haemophiliacs the absolute number of CD4+ cells did not change during the observation period. In these long-term non-progressor patients no clinical symptoms and no increased neopterin levels were observed. T cells subset and neopterin measurements were found to predict the development of AIDS. AIDS developed only in those patients who exhibited both a CD4+ cell count of < 350/microliter and a serum neopterin concentration of > 20 nmol/l. A negative correlation was observed between the percentage of activated. DR+ T lymphocytes and the CD4+ cell counts.