BACKGROUND: The mass media frequently cover health related topics, are the leading source of information about important health issues, and are targeted by those who aim to influence the behaviour of health professionals and patients. OBJECTIVES: To assess the effects of mass media on the utilisation of health services. SEARCH STRATEGY: We searched the Cochrane Effective Practice and Organisation of Care Group specialised register, MEDLINE, EMBASE, Eric, PsycLit, and reference lists of articles. We hand searched the journals Communication Research (February 1987 to August 1996), European Journal of Communication (1986 to 1994), Journal of Communication (winter 1986 to summer 1996), Communication Theory (February 1991 to August 1996), Critical Studies in Mass Communication (March 1984 to March 1995) and Journalism Quarterly (1986 to summer 1996). SELECTION CRITERIA: Randomised trials, controlled clinical trials, controlled before-and-after studies and interrupted time series analyses of mass media interventions. The participants were health care professionals, patients and the general public. DATA COLLECTION AND ANALYSIS: Two reviewers independently extracted data and assessed study quality. MAIN RESULTS: Seventeen studies were included. All used interrupted time series designs. Fourteen evaluated the impact of formal mass media campaigns, and three of media coverage of health related issues. The overall methodological quality was variable. Six studies did not perform any statistical analysis, and seven used inappropriate statistical tests (ie not taking into account the effect of time trend). All of the studies apart from one concluded that mass media was effective. These positive findings were confirmed by our re-analysis in seven studies. The direction of effect was consistent across studies towards the expected change. The pooled effect sizes for studies assessing the impact of mass media on similar aspects of care revealed an effect upon the utilisation of health services that could not be explained by chance alone, ranging from -1.96 (95%CI -1. 19 to -2.73) for campaigns promoting immunisation programmes, to -1. 12 (95%CI -0.49 to -2.36) for those concerning cancer screening. REVIEWER'S CONCLUSIONS: Despite the limited information about key aspects of mass media interventions and the poor quality of the available primary research, there is evidence that these channels of communication may have an important role in influencing the use of health care interventions. Those engaged in promoting better uptake of research information in clinical practice should consider mass media as one of the tools that may encourage the use of effective services and discourage those of unproven effectiveness.