The level of alanine aminotransferase (ALT) in blood donors has been related to the frequency of posttransfusion hepatitis in recipients. Sixty-seven donors with elevated ALT levels were evaluated to define the duration and significance of the elevation. The ALT level remained elevated in 41 donors (61%) for a mean interval of 9 months. The ALT level was greater than the aspartate aminotransferase in all of the donors. Alcohol intake did not correlate with ALT level. Donors with persistently elevated ALT levels had a significantly higher mean percent ideal body weight (128 +/- 3.9) than donors whose ALT level became normal (116 +/- 3.1). Nine donors with elevated ALT levels for at least 6 months had needle biopsies of the liver. Seven had prominent fatty vacuolization of hepatocytes without evidence of alcoholic hepatitis. One biopsy demonstrated chronic persistent hepatitis. No other cause for the elevated ALT levels could be identified. An overweight male donor with an isolated ALT elevation may need no further investigation unless clinical evaluation suggests a source of liver injury.