Mutations of the retinoblastoma gene, most of which cannot be detected by conventional Southern blotting, are known to cause both the nonhereditary and hereditary forms of retinoblastoma and have been implicated in the development of other cancers. Nonhereditary retinoblastoma is caused by a somatic mutation. Hereditary retinoblastoma is caused by a germ-cell mutation, most often a new one, and thus there is usually no family history of the disease. Unlike patients with the nonhereditary disease, those with the hereditary form are at risk for additional retinoblastomas, and their progeny are at risk for the tumors. We used a sensitive technique of primer-directed enzymatic amplification, followed by DNA sequence analysis, to identify mutations as small as a single nucleotide change in tumors from seven patients with simplex retinoblastoma (with no family history of the disease). In four patients the mutation involved only the tumor cells, and in three it involved normal somatic cells as well as tumor cells but was not found in either parent; thus, these mutations appeared to be new, germ-cell mutations. In addition, we found point mutations in cells from a bladder carcinoma, a small-cell carcinoma of the lung, and another retinoblastoma. We conclude that the technique that we have described can distinguish hereditary from nonhereditary retinoblastoma and that it is useful in risk estimation and genetic counseling.