The belief that another person shares your appreciation for humor is likely to exert a powerful effect on interpersonal attraction because sense of humor is a highly valued quality in others, and an important personal characteristic. In an attitude similarity-interpersonal attraction laboratory experiment, participants were led to believe an unseen stranger had many, or very few, similar attitudes. In addition, participants were allowed to interact, via an intercom, with the stranger, in order to relate a favorite joke. The stranger responded positively or neutrally to the joke. Although both attitude similarity and response to the joke influenced ratings of interpersonal attraction in the expected direction, the positive response to the joke was sufficiently powerful to overcome attitude dissimilarity. A dissimilar stranger who responded positively to the joke was more attractive than a similar stranger who responded neutrally. The results are interpreted as indicating that some dimensions of similarity are more important than others, and that humor appreciation may be an especially critical dimension.