The leukemic transformation of multiple myeloma (MM) is a rare phenomenon and warrants investigation, as it changes the prognosis of the disease. No specific biomarkers have been identified to predict the progression of MM to plasma cell leukemia (PCL). PCL, the most aggressive variant of monoclonal gammopathies, is defined by the International Myeloma Working Group (IMWG) as the presence of more than 5% plasma cells in the blood count. The blood count of patients with MM or PL sometimes shows “false” lymphocytosis, linked to the presence of plasma cells. Cytological monitoring of MM becomes necessary, through careful examination of blood smears, in order to detect rare circulating plasma cells and erythrocyte rollers at an early stage. Through these observations, we wanted to show the importance of qualitative analysis of blood counts or blood smears in monitoring patients with MM in order to diagnose early any possible progression to plasma cell leukemia.