Haematogen osteomyelitis is mostly found in children and adolescents. In western Europe acute haematogen osteomyelitis (AHOM) is a rare disease. This is the cause why AHOM is often diagnosed with delay. The treatment usually is an antibiotic medication and/or surgical interventions. Uncharacteristic pain of extremities in children should always consider the diagnosis of acute osteomyelitis. Investigation should include conventional X-rays, ultrasounds or MRI to prevent the spreading of infection. In cases of multifocal infection radionuclide imaging should be undergone. Differential diagnosis should always include malignant tumor. If under treatment of antibiotics the clinical signs of illness do not decrease within 24 h surgery with fenestration of the involved bone, debridement and local application of antibiotics is indicated. In unusual cases or in cases with clinical signs of AHOM but no bacteria specification a malignant tumor has to be excluded.