The haemostatic system was investigated in 26 women taking cyclically administered triphasic combined oral contraceptives for the first time during their first six cycles. Fourteen women received Logynon (mean dose 32.4μg ethinyloestradiol, 92pg progestagen) and 12 received SHD 415G (Schering) which contains a mean dosage of 32.4μg ethinyloestradiol and 78pg gestodene, a recently developed progesterone. The Logynon group showed a significant increase (p<0.005) in fibrinogen (pre-mean 284.4 g/1; after 1 cycle 347.3 g/1, after 6 cycles 318.6 g/1) , factor VII (65.8 u/1 to 73.9 u/1 to 83.2 u/1), factor XII (1.74 u/1 to 2.41 u/1, to 2.25 u/1), plasminogen (100.9 u/1 to 135.1 u/1 to 126.3 u/1); decrease in ATIII (115.9 u/1 to 103.1 u/1 to 93.4 u/1) but no significant change in factor X (98.4 u/1 to 108.9 u/1 to 102.4) or protein C (0.85 u/1 to 0.88 u/1 to 0.94 u/1) activity. The SHD 415G group showed similar changes with an increase in fibrinogen (247.9 g/1 to 330.8 g/1 to 373 .1 g/1), factor VII (63.1 u/1 to 73.1 u/1 to 90.3 u/1, factor X (98.3 u/1 to 112.0 u/1 to 124.4 u/1), factor XII (1.46 u/1, to 1.93 u/1, to 2.03 u/1), plasminogen (110.8 u/1 to 125.4 u/1 to 136.7 u/1); decrease in ATIII (113.1 u/1 to 96.3 u/1 to 89.7 u/1), but no change in protein C (0.84 u/1 to - 0.78 u/1 to 0.85 u/1) activity. These changes were apparent after the first cycle of therapy and the differences were maintained over the six cycle period. There was no increase in protein C activity despite changes in the other vitamin K dependent proteins factors VII and X. Both low oestrogen dose triphasic pills caused similar prothrombotic changes which were not modified by the new progesterone, gestodene.