Urinary neopterin is increased in less than 20% of patients with breast carcinoma. Moderately increased neopterin concentrations are also known to accompany comorbid conditions commonly observed in patients with breast carcinoma, for example, diabetes mellitus or complications of atherosclerosis. In the present study, we evaluated the effect of the presence of comorbid conditions on urinary neopterin. A trend for higher neopterin concentrations was observed in patients with most of the comorbid conditions, but significantly higher neopterin was observed only in patients aged 70 years or older and in a heterogeneous group of patients with comorbidity other than diabetes mellitus, thyroid disorder, hyperlipidaemia, cardiac disorder or other malignancy. Significantly higher neopterin levels were noted in patients with two or more comorbid conditions. In conclusion, present data demonstrate an association between systemic immune activation reflected in increased urinary neopterin concentrations and age or presence of comorbid diseases in patients with breast carcinoma. A cumulative effect was observed with the presence of two or more comorbid conditions resulting in significantly increased urinary neopterin. These observations should be taken into account when interpreting the changes of parameters of systemic immune and inflammatory response in patients with breast carcinoma.