In 29 chronic obstructive pulmonary disease (COPD) patients and nine lung healthy volunteers, above the age of 50 years, ventilation defects were examined by (81m)Kr planar scintigraphy and (81m)Kr single photon emission computed tomography (SPECT) to investigate if SPECT adds information regarding size and extent of visually scored ventilation defects, and to correlate the extent of defects obtained from the two imaging settings with standard pulmonary function tests performed in these patients/volunteers. For testing the reproducibility of the visual defect score of (81m)Kr scintigraphy additionally 13 patients suspected for pulmonary embolism or lung cancer were included. Each series of planar or SPECT studies were read for the extent (% abnormal lung) and severity (0-3) of ventilation abnormalities. Seventeen scans were read twice for reproducibility studies. The extent of ventilation defect assessed by (81m)Kr SPECT was higher than by (81m)Kr planar (slope of regression line 0.60, P<0.0001), likewise severity score (rank signed test: P<0.0001). Correlation between ventilation inhomogeneity and pulmonary function test (residual volume and T(L,CO)) in the COPD group revealed only significance for the SPECT acquisition. We found good reproducibility of visual assessment of ventilation defect extent (correlation: 0.97, P<0.0001) and severity (Kappa 0.62). In conclusion, visual scoring of extent and severity of ventilation defects was reproducible. Ventilation defects were better demonstrated with SPECT than planar imaging. The correlation to pulmonary function was better with SPECT than planar imaging.