Female sex-steroid hormones may play an important influence in asthma. The aim of this study was to compare airway reactivity to adenosine monophosphate (AMP) in female asthmatics with natural menstrual cycles and those taking the oral combined contraceptive pill (OCP). Eighteen asthmatic subjects were evaluated. Nine subjects, mean (SEM) age, 24 (6) years, FEV1 93% (10) predicted, with natural cycles (group 1) were compared with nine subjects, age 24 (6) years, FEV1 93% (9) predicted taking the OCP (group 2). Group 1 subjects were evaluated at the follicular (visit 1) and luteal (visit 2) phases; group 2 subjects were evaluated during the week off OCP (visit 1) and at the end of the OCP cycle (visit 2). At each visit, serum progesterone and estradiol were measured. Airway reactivity to AMP was evaluated and expressed as PC20 (FEV1; mg/ml). Morning and evening peak expiratory flow rates (PEFR) were monitored throughout the study. In group 1, there was a significant increase in serum progesterone (nmol/l) and estradiol (pmol/l). (Visit 1 vs. 2): 2.5 vs. 13.5 (95% CI 2.1 to 19.9; p = 0.02) and 152.3 vs. 358.1 (95% CI 113.0 to 298.5; p < 0.001), respectively. In group 2, however, there was no increase between visit 1 vs. 2 in hormones: 0.9 vs. 1.0 and 75.7 vs. 21.8 for progesterone and estradiol, respectively. There was a significant increase in airway reactivity in group 1 during the luteal phase. Geometric mean PC20 (mg/ml) was 18.8 and 4.7 at visit 1 and 2, respectively: a 4.0-fold difference (95% CI 1.25 to 13.03; p = 0.03) amounting to two doubling doses. In contrast, there was no change in PC20 in group 2. Geometric mean PC20 was 23.5 and 21.4: a 1.06-fold difference (95% CI 0.41 to 2.78; p = 0.83). In group 1, morning and evening PEFR (l/min) were significantly different at both visits: at visit 1 (A.M. PEFR vs. P.M. PEFR) 403 vs. 430 (95% CI 5 to 50; p < 0.001) and visit 2, 415 vs. 439 (95% CI 1 to 46; p < 0.001). In group 2, there was no significant difference in diurnal PEFR variability at both visits; 411 vs. 417 at visit 1 and 413 vs. 427 at visit 2. In conclusion, asthmatic patients receiving the OCP had attenuated cyclical change in airway reactivity as well as reduced diurnal PEFR variability, which was associated with suppression of the normal luteal phase rise in sex-hormones.