Stress fractures of the patella were diagnosed in 34 cats with a mean age of two years and five months. Eighteen of the cats had bilateral fractures with a median interval of three months between fractures. All the fractures were transverse and occurred in the proximal aspect or base of the patella with no evidence of trauma. In 43 of the patellae there was radiographic evidence of sclerosis. Repairs with a pin and tension band in 18 cats resulted in a further fracture or failure of fixation in 86 per cent of the cases that were followed up. Ten of the cats had suffered fractures of other bones at different times to the patellae fractures. In the majority of the cats the fracture formed a functional non-union but some cats remained stiff and lame.