Laboratory investigations are an integral part of evaluation of a child with rheumatic disease. In rheumatic diseases markers of inflammation like CRP and erythrocyte sedimentation are usually raised. In addition, various autoantibodies like anti-nuclear antibody, rheumatoid factor, anti-neutrophil cytoplasmic antibody help in diagnosis of systemic autoimmune rheumatic diseases. Synovial fluid analysis is useful in evaluating a patient with monoarthritis. The lab tests should not be done indiscriminately and choice of test should be based on provisional clinical diagnosis. A high pre-test probability increases the utility of test. Tests with high sensitivity should be used for screening whereas tests with high specificity should be used for confirmation of diagnosis. Finally, mere presence of an abnormal test does not mean disease.