This systematic review analyzes 24 mass media campaigns' effect on HIV knowledge, attitudes and behaviors. The studies were published between 1990 and 2004 about developing countries and compared outcomes by reviewing pre-and post-intervention data; intervention vs. control groups or post-intervention data across levels of exposure. The outcomes most frequently reported were condom use (17 studes), knowledge of HIV transmission (15), decreased risky sexual behavior (eight), perceived risk of contracting HIV (six), interpersonal communication about AIDS or condom use (six), self efficacy to negotiate condom use (four), and abstention from sexual beharior (three). Varying results were achieved in these studies, but generally effect sizes were small to moderate. For two of the seven outcomes, knowledge of HIV transmission and decreased risky sexual behavior, at least 50% of the studies showed a positive impact of mass media. More rigorous evaluation on comprehensive programs is required to definitely answer whether mass media affects HIV/AIDS-related behavior.