The aim of the present study was to investigate whether the early changes in the immune system observed after ABMT would persist over years. Eighty-five patients with malignant lymphoma were treated with ABMT in Norway from 1987 until 1993. Of the 46 patients in CR by 1997, 36 were enrolled in our study. Median time from ABMT was 5 years (4-10 years). Immunophenotyping showed an increase in the median number of B cells (0.35 x 109/l in patients vs 0.28 x 109/l in controls), and a decrease in T cells (1.08 vs 1.35 x 109/l). Furthermore, a lower median count of CD4+ T cells (0.54 x 109/l in patients vs0.87 x 109/l in controls) resulted in reduced CD4/CD8 ratios (0.8 in patients vs 1.6 in controls). The subgroup of CD4+ T cells expressing the 'naive' phenotype CD45RA was 19.5% in patients vs 38% in controls. In contrast, the fraction expressing the 'memory' phenotype CD45RO was higher in the ABMT group (76% vs 54%). When stimulated, larger fractions of CD3+CD4+ cells in patients produced IFN-gamma (32% vs 16%) or IL-4 (7% vs 1%) compared to controls; thus a differentiation into the functionally separate subgroups Th1 and Th2, with a dominant Th2 response. Our data further suggest that the decrease in CD4+ T cell counts and the imbalance between CD45RA+ and CD45RO+ subsets persists 4-10 years after ABMT.