As part of an evaluation of the 1990-91 anti-tobacco media campaign carried out by the California Department of Health Services, a study was conducted among 417 regular smokers who had quit during the period of the media campaign. In brief telephone interviews, all respondents identified up to three events or experiences that had influenced them to quit. In response to uncued questions, 6.7 percent of those interviewed indicated that they had been influenced to quit by an advertisement they had seen or heard on radio, television, or billboards. In response to direct questions about the media campaign, 34.3 percent of the respondents indicated that the media campaign's advertisement had played a part in their decision to quit. Applying the 6.7 percentage to the number of Californians who quit smoking in 1990-91, it can be estimated that for 33,000 former smokers, the anti-tobacco media advertisements were an important stimulus in their quit decision. Multiplying the 34.3 percent by the number of former California smokers who quit in 1990-91, the estimate of former smokers for whom the media campaign's advertisements played at least some part in their decision to quit rises to 173,000 persons. While causal attributions from such investigations should be made with caution, the evidence suggests that the 1990-91 campaign did influence substantial number of smokers in California to quit.