Piriformis syndrome (PS) is defined by a loose cluster of symptoms arising from entrapment of one or both divisions of the sciatic nerve as they pass the sciatic notch. This paper presents a method of using the H-reflex as an aid in the diagnosis of PS. Forcible pressure from the piriformis muscle on the sciatic nerve can be induced by internal rotation of an affected limb in an adducted, flexed position. This pressure is reflected in a delay of the H-reflex. The length of delay seen in 39 legs of 34 patients who met the criteria for PS is compared with that seen in 13 unaffected contralateral limbs and 14 limbs from able-bodied subjects. Mean delay of H-reflex was 2.66 msec for affected legs and .36 msec for the combined control groups (t = 7.45, p less than .001). There were no differences in the H-reflexes themselves between groups. Physical therapy aimed at reducing mechanical impingement was successful in 11 of 12 patients on followup at three to nine months.