Activated protein C (APC) resistance, both in its congenital form, due to the factor V Leiden mutation, and in its acquired form, are important risk factors for systemic venous thrombosis. In view of the suspected thrombotic aetiology of some cases of recurrent miscarriage, the prevalence of APC resistance was determined among 1111 consecutive Caucasian women with a history of either recurrent early miscarriage (three or more consecutive pregnancy losses at <12 weeks gestation; n = 904) or a history of at least one late miscarriage (>12 weeks gestation; n = 207). A control group of 150 parous Caucasian women with no previous history of adverse pregnancy outcome was also studied. Acquired APC resistance was significantly more common among both women with recurrent early miscarriage (8.8%: 80/904; P = 0.02) and those with late miscarriage (8.7%: 18/207; P = 0.04) compared with controls (3.3%: 5/150). In contrast, the frequency of the factor V Leiden allele was similar among (i) women with recurrent early miscarriage (3.3%:60/1808; 58 heterozygotes and one homozygote), (ii) those with late miscarriage (3.9%:16/414; 14 heterozygotes and one homozygote) and (iii) the control group (4.0%:12/300; 12 heterozygotes). Acquired but not congenital APC resistance (due to the factor V Leiden mutation) is associated with both early and late miscarriage.