To evaluate the relevance between interstitial lesions and acute vascular rejection of renal allograft, we examined 129 renal allograft biopsy samples obtained from 60 transplant patients within 90 d post-transplantation. Plasma cells, eosinophils and polymorphonuclear cells (PMNs) in the cortical interstitium were counted, respectively, and the number of each cells per 10 fields at a magnification of 200x was calculated. Then, the existence of interstitial edema was judged. We categorized 129 biopsy specimens into specimens before (Pre), on (R) and after acute rejection (Post). An increased number of eosinophils (mean +/- SE; 15.4 +/- 9.2 per 10 fields) was observed in R rather than in Pre (0.6 +/- 0.3, p < 0.05). The increasing number of plasma cells depends on a time course following the onset of acute rejection. The number is significantly larger in acute vascular rejection (4.3 +/- 1.1) than in non-vascular rejection (1.3 +/- 0.4, p < 0.01). In living donor transplants, the number of PMNs in patients before vascular rejection (median, 16.7 per 10 fields; range, 5.0-20.0 per 10 fields) was significantly larger than in normal or borderline patients (2.2; 0.0-36.7, p < 0.01). The incidence of interstitial edema in patients before vascular rejection (55.6%) was significantly higher than in normal or borderline patients (8.0%, p < 0.01). This study suggested that acute vascular rejection is relevant to interstitial lesions, such as for a plasma cell, eosinophil and PMN infiltrate, and edema. Also, these lesions seem to be the indicator of acute vascular rejection. These findings may contribute to the refinement of histological diagnosis of acute vascular rejection.