To assess the role of a single maternal serum progesterone measurement in the immediate diagnosis of early pregnancy failure and in the long term prognosis of fetal viability. A prospective comparative study of women presenting with vaginal bleeding and abdominal pain in early pregnancy. The comparison group was defined retrospectively as women who presented with abdominal pain without history of, or the subsequent occurrence of, vaginal bleeding and whose pregnancies continued to viability. The study groups were defined retrospectively as threatened-continuing, non-continuing (including blighted ovum, missed abortion, incomplete and complete abortion) and tubal pregnancy groups, according to the outcome of the pregnancies. The emergency room at the gynaecology department of a teaching hospital. Four hundred and eighty-nine women presenting with singleton pregnancy, vaginal bleeding and/or abdominal pain in the first 18 weeks of pregnancy. The comparison group comprised 131 women without vaginal bleeding whose pregnancies continued to viability. The study group comprised 358 women with 148 threatened-continuing pregnancies, 175 non-continuing and 35 tubal pregnancies. A 10 ml blood sample was taken and pelvic ultrasonography was performed at presentation. Otherwise, conventional management was used. Progesterone levels were interpreted in accordance with the outcome of the pregnancy: comparison, threatened-continuing, non-continuing or tubal. Viability was defined as 28 weeks or more weeks of gestation. Progesterone levels were significantly lower in the non-continuing and tubal pregnancy groups than in the comparison and threatened-continuing groups (P < 0.001 in all cases). A cut-off level at 45 nmol/1 was found to differentiate between the viable (comparison and threatened-continuing) pregnancies and the abnormal (non-continuing and tubal) pregnancies with 87.6% sensitivity and 87.5% specificity. A single serum progesterone measurement taken in early pregnancy is valuable in the immediate diagnosis of early pregnancy failure and the long term prognosis of viability.