Background: There is no reliable objective test for the diagnosis of myofascial pain syndromes. The aim of this study was to evaluate the usefulness of a pressure algometer for the diagnosis of the trigger points and for the evaluation of the treatment in myofascial pain syndromes (MPS). Methods: Twenty female patients with clinical MPS of shoulder were included in this study. Pressure pain thresholds were measured by a pressure algometer at three different sites including the trapezius, supraspinatus and infraspinatus before, and then the 1st, 3rd and 7th days after TPI. Results: Mean pressure pain thresholds were lower in patients with MPS in than normal volunteers in all the examined skeletal muscles. Mean pressure pain thresholds in patients with MPS were increased significantly after TPI in all the examined skeletal muscles. Conclusions: Pressure algometer can be used as relatively objective diagnostic tool for locating trigger points and to quantify the effect of TPI in MPS. However, more investigation is necessary.