The ability of heavily and lightly piliated Proteus mirabilis to infect the renal parenchyma was compared in a model of hematogenous pyelonephritis. Cortical abscesses occurred in 13 of 24 rats injected with lightly piliated P. mirabilis but in none of 24 rats challenged with heavily piliated organisms (P less than 0.001). Lightly and heavily piliated organisms were cleared from the vasculature equally rapidly and were also delivered to the kidney in equal numbers. During the first 24 hr, however, titers of the lightly piliated organisms in the kidney increased by 4 logs, whereas the heavily piliated P. mirabilis were virtually all eliminated. Pili are believed to mediate attachment to cell surfaces, and heavy piliation has been correlated with enhanced virulence when P. mirabilis invades the kidney across the pelvic mucosa. The results in this study suggest, however, that pili may adversely affect bacterial survival within the renal parenchyma.