Neuropsychological and neuroimaging studies strongly suggest that the inferior parietal cortex is important for calculation. However, the evidence from neuroimaging experiments for a left hemispheric dominance in calculation is not as clear as one would expect from the studies of patients. Often a concomitant activation of the homologous inferior parietal region of the right hemisphere is reported in the same tasks. The objective of this study was to replicate basic findings of acalculic patients and to investigate discrepancies between data from patients and results from neuroimaging studies in an addition task. Repetitive transcranial magnetic stimulation (rTMS) was applied over inferior parietal areas and the adjacent intraparietal sulcus (IPS) while subjects solved double-digit addition tasks. From studies of acalculic patients it was hypothesised that left hemispheric rTMS stimulation should result in longer reaction times (RTs) in the addition task. On addition trials without TMS subjects showed the classical problem size effect with longer RTs the larger the sum of the two operands. Magnetic stimulation over left inferior parietal areas disrupted performance significantly. The effect was specific to the left hemisphere stimulation. There was no increase in RTs for rTMS stimulation over the right hemisphere.