In this study we evaluated the temperament characteristics of a group of 13 subjects with Williams-Beuren syndrome (WBS) and compared the results to the findings in a control group of 13 individuals with the same degree of mental retardation of different etiology. On the different subscales of the Dutch adaptation of the Parent Temperament Questionnaire no statistically significant differences between the WBS and the control group were noted. An easier temperament was noted in the control group, and we also found greater intensity, less persistence and lower treshold in WBS subjects. The present findings indicate that the "specific" behavioural phenotype in WBS patients is apparently more related to mental retardation itself than to the underlying genetic defect. Further studies on a large group of WBS patients and mentally retarded control group are needed to confirm these findings.