Respiratory diseases of cattle, particularly in young animals, represent the most important health and economic problem of cattle rearing. The aim of the present study was to evaluate the changes of selected blood indices in 25 calves aged 2–9 months suffering from etiologically undifferentiated chronic respiratory diseases. Blood samples were analysed for haematological indices and selected serum biochemistry variables. The results found in sick animals were compared with results from 25 healthy animals of the same age, housing and feeding system. Significant differences in means between the groups of clinically healthy and sick calves were found in 13 out of 24 evaluated indicators. In sick animals we found significantly higher mean concentrations of haemoglobin and total number of white blood cells (P < 0.05 and P < 0.01, respectively), as well as higher mean activity of AST and LDH (P < 0.01), concentrations of total proteins and total immunoglobulins (P < 0.001). Significantly lower mean values were recorded in the serum concentrations of albumin, creatinine and glucose (P < 0.001), as well as in the concentration of Mg (P < 0.01), P (P < 0.001), Fe and Zn (P < 0.05). The presented results suggest the effect of respiratory diseases in calves on several changes of haematological and selected serum biochemical indicators. They indicate that respiratory diseases did not lead only to direct disturbance of gas exchange and acid-base balance, but they also indirectly affect some other variables of blood biochemistry. Bovine, bronchopneumonia, blood, haematology, clinical biochemistry Respiratory diseases are the most prevalent disease in young cattle. Prophylactic measures, high cost of therapeutic interventions, mortality, reduced growth performance due to bovine respiratory disease (BRD), and irreversible pulmonary lesions lead to considerable economic losses (Coghe et al. 1999). Respiratory diseases in calves are associated with a wide scale of pathological findings in the lung parenchyma, as well as in the respiratory pathways. Although various forms of calf respiratory diseases are known, their clinical manifestation is frequently less differentiated. Despite the great variety of combinations of pathogens that have been implicated as causes of this disease, the clinical entity of most manifested is bronchopneumonia (Lekeux 1995). Cattle typically face two primary problems that contribute to a high incidence of BRD. First, stress associated with weaning and transportation has a negative effect on the immune system. Second, this stress typically occurs when the animal is exposed to a variety of infectious agents as a result of marketing (transporting) management procedures. Nutrition can interact with these two primary factors, most likely as a result of preweaning nutritional deficiencies or through decreased feed intake associated with stress. Low feed and thereby nutrient intake makes correction of nutritional deficiencies difficult, which could further compromise immune function and potentially increase susceptibility to infection (Cole 1996). Respiratory diseases lead to various degrees of hypoxaemia with possible subsequent tissue hypoxia. Other possible consequences include accompanying hypercapnia and respiratory acidosis (Wall 2001). Hypoxaemia and hypercapnia are the direct consequence of disturbances in basic respiratory functions, namely gas exchange. In calves suffering