It is difficult to observe human tumor progression as precursor lesions are systematically removed. Alternatives to direct observations, commonly used to reveal the hidden past of species and populations, are sequence comparisons or molecular clocks. Noncoding microsatellite (MS) loci were employed as molecular tumor clocks in 13 human mutator phenotype (MSI(+)) colorectal tumors. Quantitative analysis revealed that specific patterns of somatic MS mutations accumulate with division after loss of mismatch repair (MMR). Tumors had unique patterns of MS mutation, and, therefore, based on this model, each tumor had its own unique history. Loss of MMR occurred very early relative to terminal clonal expansion, with an estimated average of 2,300 divisions since loss of MMR and 280 divisions since expansion. Contrary to the classical adenoma-cancer sequence, MSI(+) adenomas were nearly as old as cancers (2,000 versus 2,400 divisions since loss of MMR). Negative clinical examinations preceded six tumors, independently documenting an absence of visible precursors during early MSI(+) adenoma or cancer progression. These findings further extend a window beyond visible progression since loss of MMR appears to start a genetic phase involving clone sizes or phenotypes below a threshold of clinical detection. This previously occult prologue before visible neoplasia is longer and therefore likely more important than generally appreciated.