The effects of inhaled beclomethasone dipropionate (BDP), 800 micrograms daily, on the long-term course of asthma and chronic obstructive pulmonary disease (COPD) were investigated in a prospective, controlled study, over three years. During the first two years, patients were treated with a bronchodilator only (salbutamol or ipratropium bromide). Fifty six patients (28 asthma, 28 COPD), with an unfavourable course of disease during bronchodilator therapy alone (an annual decline in forced expiratory volume in one second (FEV1) of > or = 80 ml.yr-1 in combination with at least one exacerbation.yr-1), were selected for additional treatment with inhaled beclomethasone dipropionate (BDP), 800 micrograms daily, during the third year. The FEV1 and provoking concentration of histamine producing a 20% fall in FEV1 (PC20-histamine) were assessed at six-monthly intervals. In asthma, the annual decline in prebronchodilator FEV1 of -158 ml.yr-1 during bronchodilator therapy alone was followed by a significant increase of 562 ml.yr-1 during months 1-6 of BDP treatment (p < 0.0005). During months 7-12 of BDP, the FEV1 declined slightly with -31 ml.yr-1, which was not statistically different from the annual decline before steroid therapy (p = 0.17). In COPD, the increase of 323 ml.yr-1 during months 1-6 of treatment with BDP was different from the annual decline of -156 ml.yr-1 before BDP (p < 0.05). The PC20-histamine improved by 308 doubling doses during 1-12 months of BDP in asthma (p < 0.05) but not in COPD.(ABSTRACT TRUNCATED AT 250 WORDS)