Massive localized lymphedema is a monstrous tumefactive pseudosarcoma seen in middle-aged morbidly obese adults. Since its initial description in 1998, the etiology remains unknown, although associations with trauma, surgeries, and hypothyroidism have been reported. Herein, we report the largest study of massive localized lymphedema and expand upon its clinicopathologic features. Fifty-four cases from 46 patients were retrospectively identified from the institutional archives of The Ohio State University Wexner Medical Center between 2002 and 2015. Forty-six patients (21 males and 25 females, mean age 50 years) presented with large masses developing over a 5-60-month period. The majority of patients were Caucasian (n=39). All patients were obese with a mean weight of 384.7 lb and a mean body mass index of 59.6 kg/m(2). Thirty-six patients had a history of atherosclerotic cardiovascular disease and diabetes mellitus type 2 was present in 22 patients. Eight patients had multifocal massive localized lymphedema. The sites included thigh (n=33), abdomen (n=17), suprapubic region (n=1), mons pubis (n=6), scrotum (n=2), perianal region (n=1), and right flank (n=1). Mostly, the clinical impression was benign processes, including pannus or lymphedema pseudotumor. Grossly, the mean weight was 8237 g and the mean size was 53.2 cm. Histologically, eight cases showed a unique pattern of dystrophic calcifications mimicking hyperchromatic, atypical nuclei that might lead to misdiagnosis of liposarcoma, four cases showed focal metaplastic ossification, and three cases showed multinucleated cells in addition to prototypic features of massive localized lymphedema. We report that this is the largest series of massive localized lymphedema. This is a lesion mostly seen in morbidly obese patients and the thigh is the most common site of involvement. We note a marked racial predilection for Caucasians and a tendency towards multiplicity. We suggest that obesity itself and the related metabolic syndrome have an important role in its pathogenesis.