Profiles of remodeling sites were evaluated in longitudinal sections of multiple ribs from 6 beagles with renal failure and parathyroid hyperplasia and compared with ribs from 20 controls. The dogs with renal failure had a marked increase in the number of cutting cones and closing cones. The length of the cutting cone was significantly greater than those found in controls, but there was no difference in the width of the base. There was no difference observed in the length or width or in the proportion of branching of closing cones in the two groups. In addition to cutting and closing cones, however, complex remodeling sites were also seen within the cortex. These were sites in which irregular tunnels that were wider than closing cones had intermittent segments of surface containing osteoid, osteoblasts, or osteoclasts or a mineralized surface with inactive lining cells. These complex remodeling sites were found infrequently in controls and occurred primarily near the cortical endosteal surface, where most contained hematopoietic cells and presumably represented endostealization of the cortex. In the dogs with renal failure, they were markedly increased in number, significantly greater in length, and found more frequently in the center cortex, often without hematopoietic cells.