Comorbidities are often present in adult patients treated for malignant hematological diseases. In older patients, these disabilities can have an influence on the natural course of the malignant disease, on the tolerance to treatment and clinical decision making. Moreover caring of patients with several illnesses may generate high costs. To evaluate their incidence and their influence on treatment decisions, we conducted a retrospective analysis of 330 charts of patients treated for malignant diseases in the Department of Hematology at Saint Antoine Hospital during 2003 and 2004. The median age was 61 years. Forty percent of the patients were treated for lymphomas, mainly non-Hodgkin lymphomas; 16% for myelomas, 16% for chronic lymphocytic leukemia, 16% for a myeloproliferative disorder and 8% for acute leukemia. Comorbidities were present in 84% of the patients: hypertension in 35%, coronary disease in 16%, diabetes and chronic obstructive pulmonary disease in 13%, renal failure, heart failure and arrhythmias in 10% respectively. Due to the presence of comorbidities, treatment was changed in 62/276 patients (22,46%). The diseases associated with a change were in a decreasing order: neurologic deficiency (out of stroke) (odds ratio [OR]: 4.86; 95% CI: [1.47-16.02]; P = 0.009), insulin-dependent diabetes (OR: 4.33; 95% CI: [1.40-13.31]; P = 0.01), chronic obstructive pulmonary disease (OR: 3.33; 95% CI: [1.37-8.08]; P = 0.007), renal failure (OR: 3.07; 95% CI: [1.27-7.43]; P = 0.01), coronary disease (OR: 2.89; 95% CI: [1.30-6.42]; P = 0.009) and hypertension (OR: 2.74; 95% CI: [1.39-5.38]; P = 0.003). Comorbidities are an important factor to define precisely patients with hematological malignant diseases and have to be integrated in any cost caring evaluation. Likewise, comorbidities have to be correctly assessed in oncological studies.