Toxoplasma gondii, which can cause a variety of symptoms, is a protozoan widely distributed around the world. Due to a suppressed immune response, reactivation of this disease can cause mortality. We designed a cohort study for the alterations in clinical course, laboratory findings, and presence of FeLV and FIV. We created two groups (Groups I and II). Group I contained 11 cats diagnosed with clinical toxoplasmosis. Group II included 15 healthy seronegative cats. We based our diagnosis of toxoplasmosis on the presence of the following clinical signs: a fourfold increase in immunoglobulin G for Toxoplasma gondii, presence of clinical signs, differential diagnosis of other causes and positive response to treatment with clindamycin hydrochloride. We found significant differences in erythrocyte, leukocyte, hemoglobin, and hematocrit levels between the groups (respectively P = 0.05, P < 0.001, P < 0.05, P < 0.01). The mean leukocyte counts of Group I was 37985 µL. After treatment, we determined significant differences in leukocyte (P < 0.05) and trombocyte (P < 0.05). Although all cats in Group I were found negative for FeLV and FIV, six cats had other disorders. As a result, we concluded that severe clinical signs might occur in the absence of FIV and FeLV more frequently than previously thought. Further research must be performed on the diagnostic utility of resistant and severe leukocytosis in clinical toxoplasmosis.