Multiple studies have indicated that the Joe Camel advertising campaign has been successful in marketing tobacco to children and adolescents, whereas other studies have reported that current tobacco warning messages are ineffective. To determine the importance and believability of familiar and novel tobacco warning messages with and without cartoons that were modeled after Joe Camel. Children and adolescents (N = 580) in Chicago, Ill, public schools were surveyed to determine the believability and importance of 3 cartoon tobacco warnings modeled after Joe Camel developed with the messages "Smoking Causes Lung Cancer, Heart Disease, Emphysema, and May Complicate Pregnancy" or "Smoking Kills" and the same 2 messages without cartoons. Respondents rated all 3 cartoons significantly more believable than the plain condition regardless of the message (P<.05). Furthermore, respondents rated the "Smoking Causes Lung Cancer, Heart Disease, Emphysema, and May Complicate Pregnancy" warning significantly more believable and important than the "Smoking Kills" message across all 4 cartoon conditions (walrus, penguin, bear, and no cartoon) (P<.01). Selected demographic groups found particular cartoon and warning messages more believable and/or important than others. The finding that cartoon tobacco warnings are more believable than plain warnings suggests that it may be desirable to include cartoons in future tobacco warning labels. The lower ratings of believability and importance of the "Smoking Kills" warning is a concern because similar warnings have recently been implemented in at least 2 countries (Australia and Canada) and have been considered for implementation in the United States. Arch Pediatr Adolesc Med. 2000;154:1230-1236.