Flat-chested kitten syndrome (FCKS) is a disorder in cats wherein kittens develop a compression of the thorax (chest/ribcage) caused by lung collapse. This is a soft-tissue problem and is not caused by vertebral or bony malformation. However, lung collapse can be a secondary symptom caused by bony deformity affecting the thorax such as pectus excavatum. In mild cases, the underside of the chest becomes flattened (hence the name of the condition); in extreme cases the entire thorax is flattened, looking as if the kitten has been stepped on. The kitten will appear to go from normal to flat in the space of about 2–3 hours, and will usually then stabilise. FCKS is frequently misdiagnosed as pectus excavatum due to inadequate veterinary literature or lack of experience with the condition on the