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HDR brachytherapy is a well-established technique in prostate cancer most commonly used as a single dose boost with external beam treatment. It has also been explored as sole radiation modality in schedules delivering a radical dose over 2 to 4 or more fractions. Evidence from this experience has identified a dose of 36Gy in 4 fractions, 30-32Gy in 3 fractions and 26-27Gy in 2 fractions to all achieve high biochemical relapse free survival rates with low rates of genitourinary and gastrointestinal toxicity. Following on from this experience several groups have investigated the possibility of using a single dose of HDR brachytherapy for localised prostate cancer. Using a simple linear quadratic model the equivalent dose to the previous fractionated schedules using an alpha beta value of 1.5 is around 19-20Gy. There are now several cohorts of patients with low risk prostate cancer treated with monotherapy schedules of 19Gy with several years follow up. Safety, in terms of both acute and late toxicity is good with low rates of gastrointestinal and genitourinary toxicity and preserved erectile function. However there is uncertainty around efficacy with early (2 to 3 years) biochemical progression free survival rates of 60-94%. A phase II randomised trial of 19Gy vs 27Gy confirms a lower bPFS after 19Gy with relapses seen predominantly in the dominant nodule region at presentation. Two questions emerge from this: (1) is there a subpopulation for whom single dose 19Gy HDR monotherapy offers safe effective treatment (2) is fractionation essential in prostate cancer to achieve maximal cell kill or will higher single doses be the answer.