Preoperative differential diagnosis of a swelling in the neck is mandatory for a good therapeutic planning. Indiscriminate biopsy as first approach is to be disregarded, since it can compromise the result of a radical neck dissection in cases where a metastasis of epidermoid cancer is histologically recognized. A cervical biopsy is not to be performed until a complete HNT examination (including the thyroid and major salivary glands) has excluded the presence of a primary tumor in these regions. Since every localization of the head and neck malignancies shows rather fixed patterns in its lymphatic spreading, a correlation between site of T and particular localizations of N within the neck can be found. The knowledge of the probabilities that an adenopathy in a given region of the neck correlates with a primary tumor located in a given site of the body could strongly aid the physician in detecting the latter, following a rational statistically based approach. This study was performed applying Bayes theorem for probability calculus to the maps of distribution of metastases of tumors of different site in 3,700 patients who were taken into National Cancer Institute of Milan during 7 consecutive years. The results are gathered in tables and text-figures showing the great significance of some correlations, who become more evident when the adenopathies are multiple, but who usefully direct the physician also when the adenopathy is only one.