Various isoenzymes of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) exist in human colorectal mucosa. In our last experiments we have shown that ADH and ALDH are present also in colorectal cancer cells. Moreover the activities of total ADH and class I isoenzymes were significantly higher in cancer tissue than healthy mucosa. This may suggest that these changes may be reflected by enzyme activity in the serum. Therefore, we have measured the activity of total ADH, and classes I-IV of this enzyme and ALDH in the sera of patients suffering from this cancer. Total ADH activity was measured by a photometric method with p-nitrosodimethylaniline (NDMA) as a substrate and ALDH activity by the fluorometric method with 6-methoxy-2-naphtaldehyde as a substrate. For the measurement of the activity of class I and II isoenzymes we employed fluorometric methods, with class-specific fluorogenic substrates. The activity of class III ADH was measured by the photometric method with formaldehyde and class IV with m-nitrobenzaldehyde as a substrate. Serum samples were taken for routine biochemical investigations from 52 patients with colorectal carcinoma before treatment. A statistically significant increase of class I ADH isoenzymes was found. Therefore the total ADH activity was also significantly increased. The total ALDH and the activity of other tested ADH isoenzymes were unchanged. We also observed the increasing tendency of ADH I activity in accordance with the advance of disease. The activity of class I ADH isoenzymes was elevated in the serum of patients with colorectal cancer. This activity was derived from colorectal cancer cells and probably from severely damaged liver by metastatic disease.