For evaluation of effects of intervention for voice disorders objective tools are needed. The Dysphonia Severity Index (DS) is such an objective tool. The parameters of the DSI are the highest possible frequency, the lowest intensity, the maximum phonation time and the percentage jitter. The scores on the DSI are approximately in a range of -5 till +5. The DSI is related to the perceptual evaluation of voice quality; the worse the voice quality, the lower the score on the DSI. There are no significant differences between males and females. In a population without voice disorders, age appeared to have a significant influence on the DSI: with advancing age, the DSI is getting lower, in both males and females. In repeated measures of the DSI (without intervention in between), the difference between measures has to be at least 2.5 to be a significant change. The results of measures are not dependent of the investigator. The DSI is applicable for evaluation of intervention for voice disorders, such as phonosurgery or voice therapy in groups of patients. On an individual level it seems that small changes are not really well detected with the DSI, since those changes are often within the normal variations.