Magnetic susceptibility effects in magnetic resonance (MR) imaging of normal lung parenchyma occur because of magnetic-field inhomogeneities induced by the microscopic heterogeneity of the lung. The effects on MR imaging of the lung are loss of signal from intravoxel phase dispersion (measured with T2') and a shift in the macroscopic resonant frequency from that of water toward that of air (delta v). These effects of MR imaging at 1.5 T were quantitated by measuring T2' decay and delta v at different locations in the lungs of two adult volunteers and one excised inflated human lung. The average T2' was 7 msec in the excised inflated specimen and 6.3 msec in normal in vivo lungs. There was a gravitational increase in T2' from nondependent to dependent lung. T2' increased to 35 msec in atelectatic lung tissue and to more than 140 msec in tumor. The macroscopic resonant lung frequency increased to 3.6 ppm more than that of mediastinal muscle. These values are important for developing MR pulse sequences appropriate for imaging lung parenchyma.