The relationship between chromosome anomalies and metabolic modifications in human colorectal cancers grafted into nude mice was studied. Two distinct chromosomal patterns have been demonstrated i.e., monosomic type (MT) characterized by multiple chromosome losses or deletions always involving chromosome 18, and trisomic type (TT) characterized by progressive gains of chromosomes. Grafted tumors conserve original karyotypes observed on corresponding primary tumors. Most changes involve the loss of chromosomes carrying genes encoding for enzymes of the de novo pathways and the gain of chromosomes carrying genes encoding for enzymes of the salvage pathways of nucleotide synthesis. In MT tumors the long arm (q) of chromosome 17 is frequently duplicated in association with a deletion of the short arm, forming an isochromosome 17q. The activities of 3 enzymes, thymidylate synthetase (TS) mapped on chromosome 18, thymidine kinase (TK) and galactokinase (GalK), both mapped on chromosome 17q, were studied. TS is a de novo enzyme and TK and GalK are salvage enzymes. A clear correlation could be demonstrated between tumor types and enzyme activities: MT tumors had lower TS and higher TK and GalK activities than TT tumors. These differences were too large to result from a gene dosage effect only. These data suggest that serial studies on grafted colorectal cancers give a better representation of metabolic disturbances than studies on fresh tumor samples, usually contaminated by non-cancerous cells.