In our previous studies we reported that lymphocytes from patients with asthma spontaneously produce histamine-releasing factor (HRF) in vitro. In an effort to examine whether spontaneous HRF production (SpHRF) by lymphocytes from patients with asthma is related to the state of bronchial hyperreactivity (BHR), 20 patients with mild to severe asthma were studied. Peripheral blood lymphocytes were cultured alone in a serum-free medium for 24 hours, and culture supernatant was assayed for HRF activity in two separate histamine-release tests with autologous basophils and normal basophils from known healthy donors. BHR was measured as bronchial reactivity to inhaled histamine and was expressed as a provocation concentration of histamine required to induce a 20% fall in FEV1 (PC20). The result of this study demonstrated that lymphocyte supernatant from all patients with asthma released significant amount of histamine from both autologous and normal basophils. Very high histamine release was usually induced by lymphocyte supernatant from severely ill patients who had PC20 less than 2 mg/ml. Statistical analysis demonstrated that the magnitude of the SpHRF significantly correlated (r = -0.86; p less than 0.001) with PC20. Since mast cell- and basophil-derived mediators have been implicated in the pathogenesis of BHR, high correlation between PC20 and SpHRF by lymphocytes suggests that the latter may contribute to the development of BHR. Further studies are required to disclose the exact relationship between SpHRF and BHR.