Tubular casts are found in a variety of conditions. Ultrastructural evaluation of casts has not been critically and systematically performed to define its usefulness. A total of 157 renal biopsies routinely processed for light microscopy (LM), immunofluorescence (IF), and electron microscopy (EM) were subjected to blind ultrastructural evaluation. The majority of the casts were in the distal nephron, and most of them (41.4%) were hyaline (HC). One-third (35%) of the cases showed admixed HC and granular casts (GC), and 25 cases (16%) had exclusively GC. In 7% of the cases, the morphology of the casts was distinctive enough to indicate specific composition. Four cases with red blood cell casts (5.6%) were associated with necrotizing glomerulopathy and IgA nephropathy. Four cases of myoglobulin casts were identified. Two cases with crystalized light-chain casts (1.3%) were associated with an underlying plasma cell dyscrasia. One case of acute pyelonephritis demonstrated polymorphonuclear cells casts (0.64%). A case of aminoglycoside toxicity revealed casts with myeloid bodies. Ultrastructural evaluation of casts may provide useful information that may be critical to establish or suggest a specific diagnosis.