We have analyzed two aspects concerning the relationship between mutations and cancer. Firstly, we consider the possibility that spontaneous mutations result from normal cellular processes. Errors in DNA replication, depurination of DNA, and damage to DNA by oxygen free radicals are important potential sources of endogenous mutations. Mutations produced by DNA polymerases in vitro are clustered, and consist predominantly of single-base substitutions. Analysis of termination sites during copying by DNA polymerase-alpha indicates that certain pause sites are associated with misincorporation of single non-complementary nucleotides, and thus pause sites could be associated with mutagenic hotspots. Depurination results predominantly in transversion mutations resultant from the incorporation of deoxyadenosine opposite abasic sites. Mutations resulting from oxygen free radical damage to DNA are predominantly single-base substitutions: C- > T, G- > C, and G- > T being the most frequent. In addition, there is an unusual mutation that results from oxygen free radical damage to DNA, two adjacent cytidines being substituted by two thymidines. The frequency of mutations observed in tumors appears to be much greater than that predicted from the known rates for spontaneous mutagenesis. Recent observations suggest that many human tumors contain four or more independent mutations. If these multiple mutations are each required for tumor development, then the occurrence of malignancies based on normal mutation rates would be exceedingly rare, and it may be necessary to postulate that tumors result from an increase in the rate of endogenous mutagenesis.(ABSTRACT TRUNCATED AT 250 WORDS)