Using both traditional methods and broad-range 18S ribosomal DNA (rDNA) polymerase chain reaction, we examined 2 cases of lethal cestodiasis, in which the disease agent had been poorly identified or misidentified. In one case, involving a patient with AIDS, we identified the human dwarf tapeworm, Hymenolepis nana, as a cause of aberrant metastatic larval disease. In the second case with similar pathologic abnormalities, involving a patient with Hodgkin's disease, we identified a larval cestode with a previously uncharacterized 18S rDNA sequence. A prior report of this case nearly 30 years ago, based on tissueexamination, had suggested that the parasite was a sparganum.