The conventional points in the differential diagnosis between myeloma and leukemia have been discussed. Evidence has been brought to show that these points of distinction cannot be regarded as being of fundamental nature. Instances are abstracted where cases of multiple myeloma show the various characteristics of leukemia and vice versa.

1. Leukemic features in myeloma have been shown in:

a. diffuse infiltration in multiple myeloma without circumscribed tumor formation and without any gross bone destruction;

b. extraskeletal visceral myelomatous spread involving the kidney, spleen, lymph nodes, etc.;

c. invasion of peripheral blood in myeloma—occasional myeloma cells (corresponding to the aleukemic forms of leukemia) may frequently be found in concentrated smears, even though they may be missed on routine examination; however, massive invasion of peripheral blood is rare;

d. increased uric acid content of the blood and elevated basal metabolism, characteristic of leukemia, frequently seen also in myeloma;

e. occurrence of myeloma in youth;

f. symptomatology of multiple myeloma at times not referable to the osseous system.

2. Myeloma features in leukemia have been shown in:

a. skeletal involvement in leukemia;

b. very rare medullary forms of leukemia (without visceral involvement);

c. occurrence of Bence-Jones proteinuria or

d. hyperproteinemia with hyperglobulinemia in rare cases of leukemia;

e. instances when the symptomatology of leukemia was referable to the osseous system.

3. Coexistence of multiple myeloma and leukemia is reviewed from the literature, and a case is reported of extensive mixed lymphocytic and plasma cell infiltration.

In conclusion, the difference between myeloma and leukemia, as far as the listed conventional distinguishing features are concerned, is merely one of incidence: what is rare in one disease, is common in the other, and vice versa. Multiple myeloma is in all probability a leukemia of plasma cells.

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