Prescribing anastrozole instead of tamoxifen increases initial adjuvant drug costs but there is an eventual saving as fewer patients will relapse. The effect of this saving depends on an accurate understanding of the cost of breast cancer relapse. We identified 232 patients relapsing between March 2000 and 2005. Seventy-seven were randomly selected for analysis of their entire hospital and community management costs from the date of relapse until death, or the end of the evaluation period (01/01/07). The mean cost per patient was 25,186 pounds (95% CI 13,705 pounds- 33,821 pounds). The median survival from time of relapse was 40.07 months (range 0.5-73 months) and median total cost per patient was 31,402.62 pounds. Equating this figure with the difference in relapse rate (4.1%), initial drug cost (4,773 pounds) gives an extra cost of 17,244 pounds/life year saved. This was the first adjuvant cost effectiveness analysis which included the community management activity of a subsequent relapse.