We present optical spectroscopy data that allowed a measurement of the redshift for the X-ray selected BL Lacertae object 1517+656 .
With a redshift of z = 0.702 this object has an absolute magnitude of M _ { B } = -26.4 and is also an extremely powerful radio and X-ray source .
Although being a high frequency peaked BL Lac , this object is one of the most luminous BL Lac objects known so far .
Being also a candidate for gravitational lensing , this object is of high interest for the BL Lac research .
Assuming several cosmological models and a realistic redshift for the lensed object , we find that 1517+656 has a mass > 2 \cdot 10 ^ { 12 } M _ { \sun } and a high velocity dispersion > 350 { km sec ^ { -1 } } .