The interpretation of chest X-rays of patients being treated for tuberculosis or sarcoidosis was investigated. A panel of five radiologists or chest physicians read a series of chest films initially placed in the correct temporal sequence. After 3 to 4 weeks, readers were asked to reexamine the same X-rays which had been placed in the reverse sequence. Readings were identified as consistent when the reader correctly put the reverse reading (improvement versus deterioration) on X-rays shown in reversed order. Proportions of consistent readings were 36, 40, 52, and 44 percent for each pair of adjacent X-rays. When no change was reported in a pair of films, readings were usually consistent. When these unchanged pairs were removed from the analysis, consistency readings of the remaining adjacent films fell to 0, 21, 18, and 33 percent. Interreader variability was also identified with one radiologist exhibiting 65 percent consistency, and another exhibiting only 10 percent. The authors conclude that inconsistent assessment of radiographic change in pulmonary tuberculosis and sarcoidosis is greatly increased when the order and pair position within order are reversed relative to the proper chronological sequence.