Aneurysms, mostly saccular, of the medium-sized muscular arteries are frequently encountered in classic polyarteritis nodosa, whereas their occurrence in smaller arteries is unclear. The objective of this study is to clarify the three-dimensional morphology of the small-sized muscular arteries involved in microscopic polyangiitis (MPA). Six autopsy cases of MPA of the acute inflammatory stage were chosen. Using serial paraffin-embedded sections of the kidney, vasculitic lesions observed in the interlobular arteries were three-dimensionally reconstructed. All of the 19 lesions showed microaneurysms, of which 18 were sausage-shaped and the other was saccular. In the former type, average outer diameter at the most expanded point was 235.4 +/- 83.8 microm (mean +/- SD), which was 2.78 +/- 0.73 times that of an uninvolved adjacent arterial diameter. The major axis-minor axis ratio in a cross-aneurysmal section was 1.17 +/- 0.16, indicating rather regular centrifugal expansion of the aneurysm. The aneurysmal length was 742.7 +/- 254.8 microm. These sausage-type aneurysms showed whole circumferential vasculitic involvement, whereas the saccular-type aneurysm contained an uninvolved arterial portion. In both types, the luminal spaces showed similar three-dimensional features to the outer surface of microaneurysms It was concluded that the interlobular arteries of the kidneys in MPA were characterized by formation of microaneurysms, most of which were sausage-shaped