A B0 mapping of highly inhomogeneous fields is introduced. The method acquires refocused echo signals generated by a missing-pulse steady-state free precession (MP-SSFP) sequence with three-dimensional phase-encoding gradients. A B0 map is calculated from the refocusing and dephasing of spin phase around the echo centers. Validation was performed in a phantom experiment conducted with a permanently inhomogeneous field produced by mounting a head gradient coil at 36 cm out of the isocenter of a 90-cm 4T magnet. Using the measured B0 field map, we demonstrate correction of image distortion caused by the extremely nonlinear inhomogeneous B0 field.