A total of 500 consecutive women (mean age 32.9 years; SD 5 years) presenting with a history of recurrent miscarriages (median 4; range 3–17) were investigated for the presence of antiphospholipid antibodies (APA), polycystic ovaries (PCO), hypersecretion of luteinizing hormone (LH) and chromosome abnormalities in order to detect an underlying cause of their pregnancy losses. All women had details of their previous reproductive history, investigations and treatment documented: 76% of the women had experienced only early pregnancy losses (miscarriage <13 weeks gestation); 32% had a history of subfertility; and significant parental chromosome rearrangements were present in 3.6% of couples. An ultrasound diagnosis of PCO was made in 56% of women, 58% of whom were demonstrated to hypersecrete LH, based on early morning urinary LH analysis. Circulating APA were found in 14% of women. An underlying cause of recurrent miscarriage—genetic, endocrine or autoimmune—was found in >50% of couples. Women in the latter two groups are being recruited to randomized treatment trials which are discussed.