Gelatinous bone marrow transformation (GMT) is a rare disorder of unknown pathogenesis, characterized by fat cell atrophy, focal loss of hematopoietic cells, and deposition of extracellular gelatinous substances, which histochemically are mucopolysaccharides, rich in hyaluronic acid. To elucidate the conditions in which GMT develops, 155 cases of GMT that had been found in our archives among 80,000 marrow biopsies were revised. GMT occurred exclusively in adults, more often in males (n = 101) than in females (n = 54). Incidence and severity of GMT lesions were maximal in young adults. The spectrum of underlying diseases was heterogeneous and age-dependent. Anorexia nervosa, acute febrile states, and AIDS in younger ages (<40 years), alcoholism and lymphomas in middle ages, and carcinomas, lymphomas, and chronic heart failure in older ages (>60 years) were most commonly associated with GMT. Seventy-eight percent of patients with GMT showed weight loss; 81% were anemic. GMT in some cases may be a reversible lesion if the underlying disorder can be eliminated. In conclusion, GMT represents an indicator of severe illness in a patient but is not specific for a particular disease. Because GMT develops in different pathologic conditions, we suggest that basic bioregulatory processes play a role in its pathogenesis.