The execution of meaningless movement sequences was studied in 12 patients with lesions of the parietal cortex in comparison to the performance of age- and sex-matched controls. Five sequences of increasing complexity had to be performed by imitation and after verbal instruction. The performance errors were qualitatively scored by means of four error categories (temporal or spatial error, addition or omission of movement components). This study examined whether the error scores depended on instruction modality, movement complexity or lesion side. Patients with left parietal lesions produced more errors than those with right parietal lesions and control subjects. While additions or omissions of movement components occurred almost equally in all groups, temporal and spatial errors were more frequent in patients with left parietal lesions only. In addition, only the latter group showed a significant increase of error rates with increasing movement complexity. There were no significant differences between the contra- and ipsilesional hand in any group. These results demonstrate that lesions in the left parietal lobe lead to a disturbed spatio-temporal organisation of movement that becomes increasingly prominent for more complex movements.