The possible association between Toxoplasma gondii infection and paediatric haematological malignancies in a group of patients and control subjects was evaluated in the present study. We performed an age-, gender- and residence frequency-matched case-control study of 101 blood cancer patients under 18 years of age, all of which were treated in Amirkola Pediatric Hospital. One hundred and thirty-eight control samples were gathered from the outpatient clinic in the hospital. All cases and controls were tested for the presence of anti-Toxoplasma IgG antibodies and then IgG-positive subjects were evaluated for IgM antibodies by enzyme-linked immunoassays. Anti-T. gondii IgG antibodies were found in 37 (36.6%) of the cases and 12 (8.7%) subjects in the control group (odds ratio 6.07, 95% confidence interval 2.963-12.437, P < 0.0001). The median and interquartile range (IQR) of IgG titre from case group (7.7 (IQR 0.25-13.5)) was higher than the control (0.2 (IQR 0.1-0.5)) (P < 0.0001). The frequency of anti-T. gondii antibodies (IgG) in lymphoblastic leukaemia (acute lymphoblastic leukaemia), Hodgkin's lymphoma and T-cell lymphoma were 33 (31.9%), 3 (50%) and 1(100%), respectively. Anti-T. gondii IgM was not detected in the IgG-positive patients in case group. In the case subjects, no significant difference was seen in the positive rates of T. gondii infection between genders (37.3% in male; 35.7% in female; P = 0.52) and ages groups (P = 0.31). This study demonstrated that T. gondii infection is prevalent in children with blood cancer. It also showed that toxoplasmosis may possibly be linked with an increased risk of childhood haematologic malignancies. Furthermore, these results may be helpful in research on blood neoplasia aetiology.