The probability of HIV-1 transmission in a small blood exposure such as a needlestick injury or an unsafe medical injection has been estimated indirectly. Now that several comparable laboratory simulations have provided data on inoculum volumes for such exposures, the epidemiological evidence supporting these estimates can be validated and qualified using dosimetry. This review of data on infective titre, viral load and injection inoculum volume compares three approaches to HIV dosimetry. Agreement across the three approaches indicates that unsafe medical injections are several times more likely to transmit HIV-1 than needlestick accidents, and that the risk remains substantial if injection equipment is wiped, rinsed or flushed prior to re-use. The 50% infective dose of HIV in blood exposures ranges from one virion (two copies RNA) in primary infection with CCR5 co-receptor using strains of HIV-1 to 65,000 copies HIV-1 RNA in blood from an asymptomatic source patient. The median transmission risks for unsafe intravenous or intramuscular injections using equipment cleaned but not sterilized after use on a symptomatic pre-AIDS patient are 1.8% (95% confidence interval [CI] 0.1-3.2%) and 0.8% (95% CI 0.1-1.4%), respectively.