A palpable mass in a woman's breast represents a potentially serious lesion and requires evaluation by history taking, physical examination, and mammography. The initial objective is to distinguish simple cysts from solid lesions, which can be accomplished with needle aspiration (Fig. 2). A solid lesion requires a firm diagnosis, and this usually calls for removing the lesion for histologic examination. A positive result on cytologic examination after aspiration is sufficiently accurate to justify one-stage diagnosis and treatment, with confirmation by examination of a frozen section obtained during the procedure. A negative or suspicious finding on cytologic evaluation is inconclusive, and outpatient biopsy is indicated. Perfection in diagnosis will require the removal of every solid mass. This can be expected to result in the biopsy of many benign lesions, but removal of many of them is desirable on other grounds. Although in some instances the probability of cancer may be exceedingly small, it is never zero. If biopsy is not recommended, the probability of cancer should be estimated so that the patient can decide whether the level of risk is acceptable to her.