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Background: Acute rheumatic fever and rheumatic heart disease are the significant nonsuppurative late sequelae of group A beta hemolytic streptococcal infection. Anti-streptolysin O titer (ASOT) is a streptococcal antibody test that is used for the diagnosis of infections and is particularly useful in the diagnosis of acute rheumatic fever. Aim of the work:To compare ASOT values in normal children to those with rheumatic fever and children with recurrent tonsillitis, and to evaluate its relationship with seasonal variations. Methods: A case-control study conducted on 300 children; 100 children with rheumatic fever, 100 with recurrent tonsillitis and 100 apparently healthy children as a control group. Children were subjected to history taking, echocardiography study and blood specimens for complete blood count, erythrocyte sedimentation rate, C reactive protein and ASOT were assessed using immunoturbidimetric assay tested by Cobas e501/502 (Roche). Human anti-streptolysin O antibodies (any type) agglutinate with latex particles coated with streptolysin O antigens. Results: Mean and SD of age in each group was 10.2±3 years for rheumatic fever group, 7.3± 3.8years for recurrent tonsillitis group and 5.4±2.4 years for control group. Rheumatic fever group included 49 males and 51 females, recurrent tonsillitis group included 58 males and 42 females, control group included 63 males and 37 females. ASOT ranged between 261-860 (mean± SD= 475.1±214.2), 210-813 (mean± SD= 257.6±56.4) and 65-820 (mean± SD= 158.8± 105.4) among those with rheumatic fever, tonsillitis and control group respectively (p=0.001). ASOT level between 400-800 IU/ml were common among patients with rheumatic fever and levels between 200-400 IU/ml were common with recurrent tonsillitis (p<0.001) with lack of diagnostic cut-off. Conclusion: ASOT levels are higher in rheumatic fever patients compared with recurrent tonsillitis and normal children. There is lack of diagnostic cut-off of ASOT, hence the value of ASOT is only within scoring system to avoid over or under diagnosis of acute rheumatic fever. An isolated high ASOT is not sufficient to diagnose or rule out acute rheumatic fever.