The purpose of this study was to determine the etiology of elevated alanine aminotransferase (ALT) in a population of asymptomatic volunteer blood donors. Subjects with an ALT value > 2.25 sd above norm (> 55 IU/liter) from the donated unit, were prospectively evaluated over a six-week interval. The subjects consisted of blood donors (78% male, 22% female) beginning basic military training at Lackland Air Force Base. Of 44,160 individuals screened, 19,877 (45%) voluntarily donated blood, 99 (0.5%) of which had confirmed ALT elevation. Of these (90 male/9 female), an associated condition or explanation was made in 12%: four with acute hepatitis B, four positive for anti-HCV, two with autoimmune disease, one with cholelithiasis and one associated with acute appendicitis. In 87 the ALT elevation could not be explained using available testing methods but may represent individual variation from a non-Gaussian distribution, be of nonhepatic origin (muscle), or of hepatic disease not detected by the diagnostic algorithm used. To increase the diagnostic yield, it is suggested that at least two elevated ALT values be established in this population over a period of time (yet undefined), before an extensive hepatic investigation is pursued.