A prospective study analysed the prognostic value of nm23-H1 allelic deletions in colorectal cancer. Of 21 patients with no evidence of distant metastases at initial operation, 11 showed nm23-H1 allelic deletions (including 1 homozygous deletion); 10 had no nm23-H1 deletions. After median follow-up of 25 months, distant metastases had developed in 8 of 11 (73%) patients with nm23-H1 deletions but in only 2 of 10 (20%) without nm23-H1 deletions (p less than 0.03). Tests with probe YNZ 22.1, near p53, showed no significant association with distant metastases. nm23-H1 may be, or may be located near, a late-acting suppressor gene in colorectal carcinoma, in which deletions may have prognostic value.