Reported are the pathologic features of 17 mesenchymal tumors documented as causing osteomalacia or rickets. Although these tumors were histologically polymorphous, they were classifiable into four morphological groups. In the first group there were ten unique tumors showing mixed connective tissue features and containing variably prominent vascular and/or osteoclast-like giant-cell components. Tumors of this group also displayed focal microcystic changes, osseous metaplasia, and/or poorly developed cartilaginous areas. The cartilaginous areas sometimes showed considerable dystrophic calcification. With one exception, all tumors of this group occurred in soft tissue and demonstrated benign clinical behavior. The single malignant tumor originated in bone, recurred locally, and metastasized to lung. The tumors comprising the remaining three groups (six tumors) occurred in bone, demonstrated benign clinical behavior, and were grouped according to their close resemblance to tumors known to occur in bone, that is osteoblastoma-like (four tumors), nonossifying fibroma-like (two tumors), and ossifying fibroma-like (one tumor).