This depends greatly on the type of exposure. The risk is, of course, much greater if the same needle is being used repeatedly by mutliple HIV+ individuals before you do. From the healthcare research standpoint, I can tell you that if a hollow-bore needle (as opposed to a sewing or suturing needle) enters a blood vessel of a KNOWN HIV + patient and then accidentally sticks you, you're risk of contracting HIV is approximately 0.3% (1 in 300). If blood from an HIV+ patient splashes you in the eye or mouth, it is approximately 0.1% (1 in 1000). This is based on accidental needlestick exposure, not passing on and using a needle from one heroin addict to another, but your question doesn't make it clear if by "injection needle" you mean that, or the needles used to inject medications in an emergency department, for example. If it is the latter, as you can see, the risk is far from "100%", as one of the answers suggests. Still, patients who come to me after needlestick exposure are some of the most terrified patients I have to deal with, so stay safe and be careful. And if you are indeed talking about sharing needles between addicts, then you are much, much more likely to contract HIV and Hepatitis B and/or C. As far as I know, there are no statistics on this situation, though.