BACKGROUND: Haemobartonella (also known as Mycoplasma) is a blood organism of cats and can cause many anemia and many clinical signs in the infected cats. OBJECTIVES: The purpose of the present study is to calculate the incidence rate of haemobartonellosis in pet cats of Tehran and study its relationship with certain factors. Moreover, some blood parameters in the infected cats were compared with non-infected ones. METHODS: 0.5 ml blood samples from cephalic veins of 120 pet cats in Tehran were prepared and after sending to parasitology laboratory and slide preparing, were stained with Giemsa. Finally, all of the prepared slides were studied with photomicroscope for presence of Haemobartonella. Meanwhile, the relationship between the infection and some factors including age, sex, breed, animal maintenance type, and color of their coats were studied with Chi square method. Moreover, some blood parameters in the infected cats were compared with non-infected by Mann Whitney test. RESULTS: Results showed that out of 120 pet cats in Tehran, 14(11.67%) were infected to Haemobartonella. Statistical analyses didn't show any significant relationship between the infection and the above-mentioned factors. Also, the results showed significant decrease in number of RBC, MCH, MCHC, NRBC, and number of Band cells compared with non-infected cats. CONCLUSIONS: Our findings showed that haemobartonellosis is one of the probable diagnoses, when we have anaemia in pet cats of Tehran.