The Swedish Family-Cancer Database was used to analyze concordant (same site) and discordant (different site) second primary neoplasms in 633,964 cancer patients diagnosed from 1958 to 1996. Cases of second malignant neoplasms were extracted from the Database if the diagnosis date of the first and second cancer differed by at least 1 month. The expected numbers of cancers were obtained by applying site-, sex-, age-, period-, residence- and socioeconomic level-specific rates in the corresponding population in the Database to the appropriate person-years at risk. The standardized incidence ratio (SIRs) of a second cancer was taken to be the ratio of observed to expected numbers of second cancers. Of all cancers, 8.5% were subsequent neoplasms (8.4% for males and 8.7% for females). SIRs for both concordant and discordant subsequent cancer were elevated in patients with cancer of the upper aerodigestive tract, colon, nose, breast, other female genitals, testis, kidney, urinary, bladder, skin, nervous system, endocrine, bone, connective tissue, melanoma, lymphoma and leukemia. The risks at some concordant sites, such as nose, squamous cell skin, bone and connective tissue in both sexes, breast in males and upper aerodigestive tract and leukemia in females, were very high (>10). At discordant sites, SIRs were less than 2 but significantly increased after all but gastric and prostatic cancer. Compared with the general population, cancer patients were at a modestly increased risk for new primary cancer after cancers at many sites, calling for attention in treatment, management and prevention.