Using a uniform analysis procedure , we measure spatially resolved weak gravitational lensing and hydrostatic X-ray masses for a sample of 18 clusters of galaxies .
We find a radial trend in the X-ray to lensing mass ratio : at r _ { 2500 } we obtain a ratio M _ { X } / M _ { L } = 1.03 \pm 0.07 which decreases to M _ { X } / M _ { L } = 0.78 \pm 0.09 at r _ { 500 } .
This difference is significant at 3 \sigma once we account for correlations between the measurements .
We show that correcting the lensing mass for excess correlated structure outside the virial radius slightly reduces , but does not eliminate this trend .
An X-ray mass underestimate , perhaps due to nonthermal pressure support , can explain the residual trend .
The trend is not correlated with the presence or absence of a cool core .
We also examine the cluster gas fraction f _ { \mathrm { gas } } and find no correlation with M _ { L } , an important result for techniques that aim to determine cosmological parameters using f _ { \mathrm { gas } } .