"She won't walk", or "He just looks like he's limping". So many things can be going on -- how do we tackle this chief complaint? You’re dreading a big work-up.  You almost want to tell the kid – please, STOP LIMPING... STOP LIMPING! S – Septic Arthritis  The most urgent part of our differential diagnosis. The hip is the most common joint affected, followed by the knee.  Lab work can be helpful, as well as US of the hip to look for an effusion,  but sometimes, regardless of the results, the joint just has to be tapped to know for sure. T – Toddler’s fracture This is usually a torque injury when the wobbling toddler pivots quickly or trips and falls.  Toddler’s fractures happen in children 1 to 3 years of age, and occur in the distal 1/3 of the tibia.  Sometimes a cast is needed, but currently there is a new trend in foregoing casting in mild cases. O – Osteomyelitis Bacteremia – from any source – can seed into any bone.  It’s not very common, but it happens: approximately 2% of children who present to an ED with limp will have osteomyelitis.  Plain films, ESR, and CRP are a fair screen to start.  For more than the casual concern, MRI is the best modality to evaluate, followed by radionuclide scintigraphy.  Although not the first choice modality, CT can show periosteal changes, such as inflammatory new bone formation or periosteal purulence. P – Perthes disease This is the famous Legg-Calvé-Perthes idiopathic avascular necrosis of the hip, usually affecting children from 3 to 12 years. They present with a slow onset pain and with an antalgic gait.  Patients will have trouble with internal rotation and abduction of the hip.  Radiographs may be initially normal.  MRI can show the culprit: decreased perfusion to the femoral head and subsequent necrosis. L – Limb-Length Discrepancy Parents may notice that he seems “wobblier” than he should be.  It may be that we are just now appreciating a congenital anomaly.  Get out the paper tape, and measure from the anterior superior iliac spine to the medial malleolus and compare both sides.   Children with limb-length discrepancy only need a non-urgent referral to pediatric orthopedics to look for congenital dysplasia of the hip, or other growth abnormalities.  Some are treated with orthotics.  Surgical options vary.  Epiphysiodesis destroys the growth plate on the unaffected side, which evens out the growth.  Other options are limb-lengthening or limb-shortening procedures. I – Inflammatory Transient Synovitis.  This is what we want them to have right?  The typical age is between 3 and 6 years, sometimes just after a URI.  To be comfortable with this diagnosis, we should have considered all of the dangerous diagnoses, the child should be well, afebrile, in minimal discomfort, and he should respond almost completely to an NSAID.  He’s the one running up and down the department after treatment – or just from sheer boredom after observation. M – Malignancy Primary bone tumors such as Ewing’s sarcoma or osteogenic sarcoma typically affect older children.  Limping, however, may be a presenting symptom of leukemia.  If you have any suspicion of the general wellness of the child, get a screening CBC, and perhaps a peripheral blood smear.  Whatever you do, make sure you get close follow up for these kids that are on your malignancy radar -- the blast crisis may not have occurred yet – but it can happen hours to days later. Plain films are insensitive for leukemic involvement of bone but they may show diffuse osteopenia, or metaphyseal bands – symmetrical high-uptake markings around the joint.  They look like stacks of paper within normal bone – you can see them also in anemia, lead poisoning, and other causes.  Also look for periosteal new bone formation, sclerosis, or lysis. P – Pyomyositis This usually presents with vague irritability, pain, and fever, and sometimes with a subacute minor trauma.  These children don’t look to well. Also think about just run-of-the-mill myositis, usually from a vira