Aims: To explore the association between cannabis and personality scores when genetic background and shared environment are controlled for. Design: The co-twin control design. This design provides a powerful method for controlling for the effects of potentially confounding familial factors that may act to predispose subjects both to cannabis use and a particular personality profile. Participants: 118 monozygotic twin pairs discordant for cannabis use. Measurements: Data on personality and cannabis use were obtained through a questionnaire survey. The Dutch Sensation Seeking List was used to assess various aspects of sensation seeking. The Amsterdamse Biografische Vragenlijst assessed extraversion, neuroticism, somatic complaints and test attitude. Findings: The affected twins scored higher on all scales than their unaffected co-twins, especially on experience seeking (P = 0.004), total sensation seeking score (P = 0.004) and neuroticism (P = 0.039). Differences were also observed when items on drug use were removed from the experience seeking scale (P = 0.037) and total sensation seeking score (P = 0.009) although these differences were no longer significant after Bonferoni correction (P < 0.005). Conclusions: Cannabis use is associated with a higher score on personality scales. This result was obtained in a sample of monozygotic twins discordant for cannabis use. Thus, at least part of the association between cannabis use and experience seeking cannot be attributed to underlying genetic or shared environmental factors that influence both personality and cannabis use and must be explained by unique environmental influences. © 2007 The Authors.