The diagnosis of acute osteomyelitis and septic arthritis is a clinical one. Acute-phase reactants, such as white blood cell (WBC) count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) are useful to help the clinicians at the time of initial diagnosis. The WBC count may be normal in up to 80 % of cases and it is not a reliable indicator. The ESR is elevated in 80 % of cases. CRP is elevated more than 80 % of cases. CRP rises rapidly within 48 hours of admission and returns to normal within a week after appropriate therapy. Its rapid kinetics is useful for follow-up of the response treatment. Patients who require surgical drainage procedures have prolonged time to normalization of CRP. PCT is a useful specific marker for predicting severe infection but its sensibility to detect bone and joint infections seems to be low.