We evaluated 100 asymptomatic blood donors with serum alanine aminotransferase (ALT) levels exceeding 0.83 mu kat/L, for evidence of liver disease or risk factors for non-A, non-B hepatitis and followed serum ALT levels for another 6 months. In 92 donors completing the study, ALT elevations occurred once in 33%, intermittently in 36%, persistently in 28%. Twenty-two donors were obese, 5 had clinical and biochemical evidence of alcoholic liver disease, and 45 drank alcohol regularly; 1 had hemochromatosis, and another, myopathy. In 22 no cause for elevated serum ALT levels was found. The presence or absence of risk of acquiring hepatitis did not correlate with the pattern of ALT elevations or the identification of another cause for the elevated ALT levels. In 92 blood donors with an initially elevated ALT level, two-thirds have intermittent or persistent elevations; most approximately 20% have no apparent cause for the elevations other than possible non-A, non-B hepatitis. These findings may be helpful in counseling and following blood donors with elevated ALT levels.