Until now, Measurement-While-Drilling (MWD) services providing formation evaluation information have been limited to natural gamma ray and resistivity measurements. A porosity sensor has been needed to complete the basic set of measurements required for quantitative formation evaluation. We describe the first MWD compensated neutron porosity measurement service (CNgSM)* which employs a dual-spaced, borehole-compensated detector system to measure neutron-capture gamma rays. CNO service, when combined with existing MWD gamma ray and resistivity services, provides the basic data necessary for calculation of water saturation from MWD logs, making it possible to replace wireline logs in many situations with resulting savings in both logging costs and associated rig time. This is particularly cost effective when drilling high angle offshore development wells and in other high cost development drilling. Making the neutron porosity measurement while drilling also limits the adverse effects of borehole washout that often plague wireline porosity measurements.