Dear Editor, Benign acute childhood myositis (BACM) is a rare but self-limited, transient disease affecting children of school-going age. It is characterised by calf tenderness and sudden onset of difficulty in walking after a viral illness.1 With the recent pandemic of swine flu, we are more likely to encounter children with similar problems. We describe a young boy who presented with classical symptoms of BACM following swine flu. An 8-year-old fit and healthy boy of Afro-Caribbean origin presented with 2 days history of pain in his calves and difficulty in walking. Over the last week he had fever, headache, cough and ‘flu-like’ symptoms which have completely subsided. Pain was getting progressively worse and was more pronounced on touching the heel of the foot on the ground while walking. There was no family history of neuromuscular disorders. He has not had any recent travel, trauma, vigorous exercise or similar episodes of leg pain in the past. On physical examination he was apyrexial with no rash or joint swelling. The throat was mildly inflamed but no exudates were seen. Bilateral calf tenderness was present but there was no localised swelling or any evidence of trauma to his lower limbs. Passive dorsiflexion of his ankles was painful. Power and reflexes were normal in both lower limbs. He was able to walk with support on his tiptoes and Gower's sign was negative. White cell count, C-reactive protein, liver function tests and renal profile were within normal range. Urine was negative for blood, leukocytes and myoglobin. Creatine kinase (CK) was elevated at 3986 IU/L. Swine-lineage Influenza A RNA (H1N1) was detected by polymerase chain reaction. Diagnosis of BACM secondary to swine flu was made. On follow-up after 2 weeks, his symptom has completely resolved and was able to walk normally. Repeat CK was within normal range (128 IU/L). BACM is a transient and rare disorder of childhood, primarily caused by inflammation of the muscles associated with a viral illness. It is thought to be distinct from diffuse myalgias that commonly precede or coincide with an influenza infection. The true incidence is not known and it affects both sexes. The clinical manifestations typically start after a few days of ‘flu-like symptoms’. Children usually present with acute pain in the calves or legs, associated with difficulty in walking. Sometimes they walk on tiptoes or can even refuse to weight bear altogether, primarily due to pain rather than true weakness of the muscles. Other muscle groups can also be affected including thighs, back, neck and proximal arm muscles.2 Often there is tenderness in the gastro-soleus complex. CK is moderately elevated and reverts to normal within 1–2 weeks. Renal and liver functions are normal but leucopoenia and transient elevation of Aspartate and Alanine Transaminases derived from muscles has been described.2 The association of BACM and influenza is well established. Historically, cases have been seen after the influenza season or the pandemic. The majority of cases follow infection with influenza B, though influenza A and other viruses like parainfluenza, adenoviruses, herpes simplex and Epstein–Barr viruses have also been linked.2–4 Pathogenesis of BACM has been a matter of debate. It occurs only after the primary infection with the virus, which may explain its rarity in adult population. The muscle damage may be related either to direct viral invasion causing injury to the muscle tissue and subsequent failure of the infecting virus to replicate once in the muscle, or due to autoimmune mechanism. It is vital to distinguish this relatively benign condition from an underlying serious pathology such as dermatomyositis, rhabdomyolysis, Guillain–Barré syndrome, polymyositis or muscular dystrophy. This can be done on the basis of a careful history and detailed clinical examination. Preceding viral infection followed by an acute onset of typical clinical findings of myositis, predominantly affecting the gastrocnemius and soleus muscles with an elevated CK level, is characteristic of BACM. Presence of myoglobinuria, recent trauma, a family history of neuromuscular disorders, chronic progression, skin rash, muscle weakness or abnormal neurological findings are not compatible with the diagnosis and warrants further investigations. Treatment is supportive as this condition resolves spontaneously but a follow-up is warranted.5,6 With the swine flu now reaching a ‘global pandemic stage 6’ according to the World Health Organization,7 it is likely that acute care physicians and paediatricians will encounter more children presenting with similar symptoms. It is important that they recognise and understand this benign clinical condition with good prognosis, in order to reassure parents and avoid unnecessary investigations.