Asbestos-related parietal pleural plaques develop slowly and are of little clinical significance. Other asbestos-related pleural reactions, for example acute exudative pleurisy and progressive pleural fibrosis, are of clinical importance. The pathogenesis of these reactions is unknown, but one hypothesis is that immunological disturbances are involved. To investigate this hypothesis a phenotypic characterization of lymphoid cells was performed in the peripheral blood of 45 patients with asbestos-related pleural lesions; 20 with pleural plaques (PP), 15 with diffuse pleural fibrosis (DPF), and 10 with benign asbestos pleural effusion (BAPE). Twenty-four healthy blood donors were used as controls. All asbestos groups together had a significantly higher percentage of B-cells than the controls. The percentage number of "helper/inducer" T-cells was significantly lower in the BAPE and DPF groups than in the control and PP groups. Thus, significant aberrations in peripheral blood lymphoid cells were found in patients with DPF and BAPE. This differed from patients with PP only who were similar to normals.