The positional accuracy of multileaf collimators (MLC) is crucial in ensuring precise delivery of intensity-modulated radiotherapy (IMRT). The aim of this planning study was to investigate the dosimetric impact of systematic MLC errors on step and shoot IMRT of prostate cancer. Twelve MLC leaf banks perturbations were introduced to six prostate IMRT treatment plans to simulate MLC systematic errors. Dose volume histograms (DVHs) were generated for the extraction of dose endpoint parameters. Plans were evaluated in terms of changes to the defined endpoint dose parameters, conformity index (CI) and healthy tissue avoidance (HTA) to planning target volume (PTV), rectum and bladder. Negative perturbations of MLC had been found to produce greater changes to endpoint dose parameters than positive perturbations of MLC (p < 0.05). Negative and positive synchronized MLC perturbations of 1 mm resulted in median changes of -2.32% and 1.78% respectively to D_95% of PTV whereas asynchronized MLC perturbations of the same direction and magnitude resulted in median changes of -1.18% and 0.90% respectively. Doses to rectum were generally more sensitive to systematic MLC errors compared to bladder. Synchronized MLC perturbations of 1 mm resulted in median changes of endpoint dose parameters to both rectum and bladder from about 1 to 3%. Maximum reduction of -4.44% and -7.29% were recorded for CI and HTA respectively due to synchronized MLC perturbation of 1 mm. In summary, MLC errors resulted in measurable amount of dose changes to PTV and surrounding critical structures in prostate IMRT.