120 Krukenberg tumors were analyzed with emphasis on their wide microscopic spectrum and resultant problems in differential diagnosis. The patients ranged from 13 to 84 years (average, 45 years) with 43% of them under 40 years. Abdominal swelling or pain usually accounted for the clinical presentation, but 17 had abnormal vaginal bleeding, 4 had virilization, and 4 had hirsutism without virilization. Ascites was present in 43% of the cases. Sixty-three percent of the tumors were documented to be bilateral, but both ovaries were not always removed or rigorously examined microscopically. The mean diameter of the tumors was 10.4 cm, and they typically had intact, bosselated external surfaces without adhesions. The sectioned surfaces were typically solid and firm to edematous to gelatinous; one third of the tumors also had cysts. Microscopic examination showed great variation from case to case and within individual neoplasms. Multiple nodules separated by normal stroma were seen in small neoplasms and focally in many larger ones. The tumors were often more cellular at their periphery and edematous to gelatinous centrally. An irregular distribution of cellular and less cellular areas often imparted a pseudolobular pattern. The cellularity of the stroma ranged from densely cellular to paucicellular; the latter regions ranged from edematous to mucoid. The overall morphology varied according to the prominence of signet-ring cells, extracellular mucin, edema, and various epithelial patterns. Signet-ring cells were numerous in most neoplasms (and by definition occupied at least 10% of the neoplasm) but were often absent or inconspicuous in significant areas of them. The signet-ring cells typically had modest but sometimes copious amounts of pale to basophilic cytoplasm; occasionally, it was eosinophilic. The signet-ring cells varied widely in their arrangement, growing singly, in clusters, forming confluent masses or pseudo-tubular arrays or lining part of all of a true tubule. Small glands and tubules were common, often resembling microcysts (when the lining cells were flattened) or Sertoli tubules; mucinous glands and cysts and medium-sized to large intestinal-type glands were also relatively common, particularly the latter. Extracellular mucin was often conspicuous and, when associated with scant acellular collagenous stroma, gave a distinctive appearance referred to by us as "feathery degeneration." Stromal luteinization was present in the tumors of the 8 pregnant patients and was seen in 14% of the nonpregnant patients. Unusual features that complicated the microscopic picture included diffuse sheets or other arrangements of mucin-free indifferent cells, squamous cells, clear cells, transitional cells, and corded, trabecular, and insular patterns. Vascular space invasion was common. Two thirds of the primary carcinomas were detected synchronously with, or subsequent to, detection of the Krukenberg tumor compounding the diagnostic difficulty posed by the cases. Two thirds of the primary tumors were in the stomach; other primary sites in order of frequency were appendix, colon, breast, small intestine, rectum, gallbladder, and urinary bladder. Our observations emphasize that the microscopic spectrum of the Krukenberg tumor is broader that often presented in the literature, in particular tubules, glands, and cysts often being present, and the wide pathologic differential diagnosis is discussed.