The computed tomographic appearance of the normal and abnormal thymus and its age-related changes have been described. However, there is little quantitative data regarding thymic morphology at the extremes of age and the value of thymic measurements, in particular thickness, to recognize thymic abnormality. Using computed tomography the thymic morphology in 309 normal patients was analyzed retrospectively, examining its appearance at the extremes of age and measuring its dimensions for comparison with similar data in 23 patients with clinically or surgically proven thymic abnormality. The study confirmed the previously reported age-related growth and subsequent involution of the normal thymus. In more than half the patients beyond the age of 40, total fatty involution of the gland occurred. When present, residual thymic tissue usually assumed a small, linear, oval, or round shape and did not produce focal alterations in the lateral mediastinal contour. Comparison of normal and abnormal glands suggests that thymic shape reliably separates normal from abnormal glands. In particular, multilobularity was never a feature of the normal gland at any age but was seen only in patients with thymic abnormality. Logarithms of the anteroposterior, craniocaudal, and transverse dimensions as well as thymic thickness were plotted against age to determine the value of quantitative measurements in detecting thymic abnormality. While thymic thickness and the logarithm of the product of transverse dimension and thickness were sensitive indicators of thymic abnormality, these were not necessary for accurate recognition of abnormality.