A Simon focus is a tuberculosis (TB) nodule that can form in the apex of the lung when a primary TB infection elsewhere in the body spreads to the lung apex via the bloodstream.^[1] Simon focus nodules are often calcified.^[1]

The initial lesion is usually a small focus of consolidation, less than 2cm in diameter and located within 1 to 2 cm of the apical pleura.^[2] In adolescence, Simon foci may become reactivated and develop into Assmann foci.^[3] Such foci are sharply circumscribed, firm, gray-white to yellow areas that have a variable amount of central caseation and peripheral fibrosis.^{[citation needed]}