Intranuclear groove is a similar phenomenon and intranuclear inclusion: it corresponds also to invagination of nuclear membrane. It is also a very important sign of papillary cancer. In a relatively great proportion of papillary cancer, this is the only nuclear sign leading to the correct cytological diagnosis. Nevertheless, the diagnostic power of grooves is less than the inclusion. The most important limitation is caused by the occurrence of grooves in oxyphilic cells - irrespectively of the origin. Although we can demonstrate an increasing prevalence of grooves in oxyphilic cells from Hashimoto's thyroiditis through oxyphilic adenoma to oxyphilic cancer, in contrast with non-metaplastic cells, we can only raise the possibility of carcinoma if grooves are present in a great number of cells.