Abstract After a well or a well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation pressure, and formation pressure gradient. Currently wireline formation testers are most commonly used to perform this testing. However, the use of these testers is sometimes limited by high operational costs, risk of tool sticking, and invasion effects on measurement accuracy. In addition, wireline formation tests interrupt the drilling process and can add days to a drilling program. This paper presents field-test results of a recently introduced Drilling Formation Tester (DFT). This tester is part of a logging-while-drilling (LWD) tool string and uses mudpulse telemetry to transmit real-time formation pressure data to the surface. Data obtained from LWD sensors are used to determine the thickness, depth, and petrophysical properties of the formation at the targeted test site in the well. Multiple pressure tests can be performed in a single bit run, thus providing the pressure gradients useful in determining oil, water, and gas contacts. Tests can be completed in less than 30 minutes, with only minimal interruption of the drilling operation. The field test results are analyzed with a recently introduced exact solution to the spherical-flow well-test equation with storage and skin. The solution is used to predict formation pressure and permeability from early- to intermediate-time pressure transients. This scope is greater than what is possible with conventional drillstem test analysis models, which do not model the early-time behavior. The new solution permits determination of storage coefficient, skin effect, formation fluid compressibility, permeability, and pore pressure. This paper will explain the concept and the basic design of the new LWD formation tester system, give examples of its applications, and discuss its advantages over conventional wireline formation testers and drillstem testers.