Vertebrate males often show breeding colours that may function as reliable signals of status in intrasexual competition. In many lacertid lizards, males show a conspicuous row of small but distinctive blue spots that runs along their body side on the outer margin of the belly. However, no study has examined the role of these blue spots. We first analysed in a field population of the Iberian rock lizard, Lacerta monticola, the relationships between number of blue spots and some morphological traits, which are known to be related to males’ fighting ability. The number of spots seems to be an character showing ontogenetic change as large (generally older) males showed more blue spots than small (generally younger) males. Males with a higher body condition also showed a higher number of blue spots. Thus, a higher number of blue spots may be used to signal size, age or body condition. Many contiguous blue spots would result in a visual artefact consisting of a continuous blue band, which might be a reliable size- or condition-dependent signal in some social contexts. We further examined in the laboratory whether male characteristics are related to dominance status. In males with similar body size or age, those with relatively larger heads were more dominant, whereas the number of blue spots was not important. Moreover, the number of blue spots in nature was not related to relative head size. Finally, we experimentally manipulated the presence and the number of blue spots of intruding males, and examined the aggressive response of resident males. Intruder individuals manipulated to cover all their blue spots received a lower amount of aggression. However, males with different numbers of manipulated blue spots received a similar number of aggressive responses. These results suggest that, during agonistic encounters, the presence of blue spots, but not their number, may elicit aggressiveness. Thus, blue spots may serve to identify an individual as an adult male, and to enhance body size of larger males.