A method of and apparatus for determining the physical posture of a patient's body, having a superior-inferior body axis, an anterior-posterior body axis and a lateral-medial body axis, in relation to earth's gravitational field. A medical device having first, second and, optionally, third accelerometers having sensitive axes mounted orthogonally within an implantable housing is adapted to be implanted with the sensitive axes nominally aligned with ideal X, Y and Z device axes correlated to patient body axes. Each accelerometer generates DC accelerometer signals having characteristic magnitudes and polarities on alignment of the sensitive axis with, against or normal to earth's gravitational field and DC accelerometer signals of varying magnitudes and polarities when not so aligned. The actual pitch and roll angles of the sensitive axes of the accelerometers in the implanted IMD with respect to the true gravitational axes are determined and the yaw angles are determined or estimated. Corrected DC accelerometer output signals are derived from the pitch, roll and yaw angles and are compared to a set of stored thresholds for each body posture to determine the actual body posture. Storage of these comparisons or the raw signals themselves can be used for histogram data to provide theraputic or research benefits also described.