Human mistakes are an important source of error in all steps of radiotherapy planning and their incidence should be investigated. As has been recognized by different authors and by the ICRU [4], the human error rate in the calculation of monitor units (MU) is relatively high. At our institute, we measured the human error rate in the calculation of MU by an independent check of the calculation. From September 1991 to June 1992 we identified and corrected 17 serious errors (deviation from the prescribed dose > or = 5%) over 890 controls (1.9%) (daily dose errors). We also found a serious global dose error rate (i.e. the errors induced on the total reference dose for the complete course of the treatment) of 1.3% (9/685) during the period November 1991-June 1992. These values suggest the importance of human errors in the calculation of MU and also confirm the validity of the independent check of MU calculation as one of the simplest ways of avoiding erroneous dose delivery by incorrect calculation of MU.