The authors study the serum lactate dehydrogenase (LDH) activity in multiple myeloma patients in relation to survival, plasma cells bone marrow infiltration, and creatinine and paraprotein serum values. Twenty five patients seen between 1979 and 1983 were studied considering two groups, one with LDH values higher than 100 IU/1 and the other with LDH values lower than 100 IU/1. Patients having more than 100 IU/1 showed a more marked bone marrow infiltration. Paraprotein concentration was higher in patients having less than 100 IU/1. No correlation was observed for the other factors considered.