A gravity study of northeaster Illinois and southwestern Wisconsin was undertaken to determine the three-dimensional geometry of the Precambrian granitic body encountered in the Illinois deep drill holes. Gravity data acquired during this study and existing observations show a marked negative gravity anomaly in the vicinity of these drill holes which is interpreted as originating from a negative density contrast between the granitic body and the intruded crystalline rocks. A fifteenth-order polynomial surface fitted to the Bouguer anomaly date (which includes only wavelengths greater than ~35 km) was chosen as best displaying the effects of the granitic body, and profiles of this surface were modeled employing a variety of techniques and assumptions. The preferred model of the causitive body has a depth to the top of 2 km, a density contrast of -100 kg m-3, and a maximum vertical extent of 5 km. An isopach map of the body constructed from the modeling places the Illinois deep drill holes on the edge of an oval-shaped mass.