Risk perceptions are important in HIV research and interventions; mixed results were found between HIV-related perceptions and behaviors. We interviewed 377 sexually active injecting drug users in China, finding mixed associations between HIV-related risk perception assessed by two general measures and two previous risk behaviors (syringe sharing: p<.05; unprotected sex: p>.05) - partially supporting the 'reflective hypothesis' that reflection on previous behaviors increases risk perceptions. When we use specific measures for risk perceptions (HIV transmission via unprotected sex with specific types of sex partner and via syringe sharing) and use behavioral intention to adopt protective risk behaviors (condom use and avoid syringe sharing totally) as dependent variables, positive significant associations were observed - supporting the motivational hypothesis that risk perceptions motivate one to adopt protective behaviors. The direction and significance of the associations of concern depends on types of measures used. It has important implications on research design, data interpretation and services.