Hemoglobin casts are thought to commonly arise from decomposing erythrocytes trapped within the usual cast matrix. They are a sign of renal parenchymal bleeding (1).
(all pictures phase contrast, x400)
When present, hemoglobin casts are easily detected due to their showy orange-brown coloration. At times it might be difficult to tell them apart from muddy-brown casts seen with acute tubular injury. Detailed examination, looking for remnants of red-cell membranes and excluding the presence of fine black granules, helps resolve this issue. Furthermore hemoglobin casts usually occur on a background of significant glomerular hematuria.
More rarely hemoglobin containing casts may be found in patients with severe hemolysis. (Sadly, I'm still waiting for it.) Obviously you would not expect to see erythrocyte remnants in these cases.
1. Fogazzi G, Ponticelli C, Ritz E. The urinary sediment: An integrated view. United Kingdom: Oxford University Press; February 3, 2000.