OBJECTIVE To investigate whether the measurement of maternal plasma creatine kinase concentration is of clinical value in the diagnosis of tubal ectopic pregnancy. METHODS A retrospective observational study covering a three month period. METHODS The gynaecological emergency service of a UK teaching hospital. METHODS One hundred and twenty women presenting with clinical suspicion of an ectopic pregnancy such that plasma human chorionic gonadotrophin (hCG) concentration was measured. METHODS Plasma hCG and creatine kinase concentrations were measured. Pregnancies were classified as ectopic pregnancy, complete miscarriage, incomplete miscarriage or ongoing pregnancy. Pregnancies were further classified as uncertain diagnosis if the diagnosis was not clear after initial investigation. RESULTS Twenty-one of the patients had an ectopic pregnancy. Diagnosis was classified as uncertain in 48% of pregnancies studied and in 54% of ectopic pregnancies during the study period. The mean plasma creatine kinase was higher in ectopic pregnancies than other pregnancies in the study group (P < 0.01) and remained higher in the subgroup of patients with uncertain diagnosis (P < 0.05) but there were significant overlaps in creatine kinase concentrations in all four clinical groups. When a cut-off point of 45 iu/l is used, the measurement of plasma creatine kinase activity has a sensitivity of 0.57 and a specificity of 0.67 for the diagnosis of ectopic pregnancy.