OBJECTIVES: Unplanned pregnancy can have a significant impact on the individuals involved. With around one third of pregnancies being unplanned, it is a significant problem for health care systems and society. The objective of this analysis was to estimate the cost of unintended pregnancy from a UK National Health Service perspective using a decision-analytic modelling approach. METHODS: The cost of unintended pregnancy was evaluated using a decision-analytic Markov model. A review of medical literature and government statistics was undertaken to identify probabilities of alternative pregnancy outcomes. The model was then used to predict the likelihood of these outcomes: ectopic pregnancy; spontaneous abortion; induced abortion; vaginal delivery; C-section; full term/pre-term delivery. Costs were derived from NHS reference costs and the literature and inflated to 2002 prices. Post-natal costs were excluded. To obtain a stable estimate of costs, we simulated a cohort of 100,000 women using oral contraceptives (OC's) over a 2-year period. RESULTS: The model predicts that in this cohort of 100,000 OC users there would be 14,211 pregnancies over two years (approximately 7% per annum). The most common outcomes were vaginal delivery (53%), elective abortion (26%) and C-section (12%). The annual cost of these events was 10.3 million pounds (GBP), with an average event cost of 1,460 GBP, varying from 2,448 GBP for C-sections to 452 GBP for spontaneous abortions. Assuming that 30% of the 764,000 conceptions that occurred in 2001 were unplanned, this would equate to a total annual cost of 335 million GBP in the UK. CONCLUSIONS: Unintended pregnancy places a large financial burden on the NHS. Investment in interventions that reduce unintended pregnancy rates are needed.