A detailed haematological study of dogs that were infected with low, moderate or high numbers of Babesia canis-infected red blood cells was performed in an attempt to elucidate the pathogenesis early after B. canis infection. Results showed that upon infection the C-reactive protein (CRP) level in plasma increased prior to the detection of parasites in the blood indicative of an acute phase reaction. The response was further characterised by fever, fibrinogenaemia, thrombocytopenia and leucopoenia. Thrombocytopenia was associated with increased coagulation time. Infected dogs also developed life threatening hypotension, and dogs that were infected with the highest dose of B. canis-infected red blood cells had to be treated chemotherapeutically. Hypotension was associated with a reduced packed cell volume (PCV). This reduction of PCV correlated with reduced plasma creatinin concentration, suggesting that the plasma volume was increased, affecting both the erythrocyte and creatinin concentration in the plasma. Importantly, the onset of the response but not the dynamics of the response was dependent on the infectious dose i.e. curves obtained with different doses of infected erythrocytes appeared to be shifted in time but had a similar shape. This indicates that infection triggered a preset inflammatory response.