In the present report we describe a recently proposed technique for the enumeration of T lymphocyte subpopulations. The study was performed on peripheral blood lymphocytes (PBL) of 21 patients with primary immunodeficiencies and 10 with systemic lupus erythematosus (SLE). With this methodology, sheep rosettes are evaluated before and after preincubation with theophylline. Previous reports have shown that cells rosetting after the preincubation (T-res) contain most of the percentages of the helper activity, whereas sensitive cells (T-sens) exert suppression. T-res and T-sens cells were compared to several clinical and immunological parameters, including the detection of lymphocytes with Fc IgM and IgG receptors. We demonstrated a significant positive correlation between T-res and and EAox IgM cells and between T-sens and EAox IgG cells. In addition, patients with Ig defects demonstrated heterogeneity regarding alterations in the proportions of T-res and T-sens cells. On the contrary, in 8 patients with SLE, there was a markedly reduced proportion of T-sens cells. Variations in the balance of subpopulations were observed after treatment with thymic hormones and levamisole. We may conclude that theophylline rosettes represent an easy technique that can be profitably used in the evaluation of T cell subpopulations in immunological diseases.