OBJECTIVE: To determine whether treatment with low dose aspirin and heparin leads to a higher rate of live births than that achieved with low dose aspirin alone in women with a history of recurrent miscarriage associated with phospholipid antibodies (or antiphospholipid antibodies), lupus anticoagulant, and cardiolipin antibodies (or anticardiolipin antibodies). DESIGN: Randomised controlled trial. SETTING: Specialist clinic for recurrent miscarriages. SUBJECTS: 90 women (median age 33 (range 22-43)) with a history of recurrent miscarriage (median number 4 (range 3-15)) and persistently positive results for phospholipid antibodies. INTERVENTION: Either low dose aspirin (75 mg daily) or low dose aspirin and 5000 U of unfractionated heparin subcutaneously 12 hourly. All women started treatment with low dose aspirin when they had a positive urine pregnancy test. Women were randomly allocated an intervention when fetal heart activity was seen on ultrasonography. Treatment was stopped at the time of miscarriage or at 34 weeks' gestation. MAIN OUTCOME MEASURES: Rate of live births with the two treatments. RESULTS: There was no significant difference in the two groups in age or the number and gestation of previous miscarriages. The rate of live births with low dose aspirin and heparin was 71% (32/45 pregnancies) and 42% (19/45 pregnancies) with low dose aspirin alone (odds ratio 3.37 (95% confidence interval 1.40 to 8.10)). More than 90% of miscarriages occurred in the first trimester. There was no difference in outcome between the two treatments in pregnancies that advanced beyond 13 weeks' gestation. Twelve of the 51 successful pregnancies (24%) were delivered before 37 weeks' gestation. Women randomly allocated aspirin and heparin had a median decrease in lumbar spine bone density of 5.4% (range -8.6% to 1.7%). CONCLUSION: Treatment with aspirin and heparin leads to a significantly higher rate of live births in women with a history of recurrent miscarriage associated with phospholipid antibodies than that achieved with aspirin alone.