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Objective: To evaluate the risk for developing second primary thyroid cancer (TC) following breast cancer (BC) and second primary BC following TC on a nationwide basis. Methods: All BC and TC Jewish females diagnosed in Israel during 1960–1998 were identified through the Israel Cancer Registry. The expected second primaries were calculated using cancer incidence rates stratified by age, country of birth and period of diagnosis among the Jewish population in Israel. Standardized incidence ratios (SIRs) were estimated using Poisson regression. Results: A total of 49,207 breast and 4911 thyroid neoplasms were identified. After the exclusion of concomitant disease (diagnosed within 1 year), 59 and 70 second primaries TC and BC yielded SIRs of 1.34 (95% CI: 1.03, 1.72) and 1.07 (95% CI: 0.84, 1.34), respectively. Younger age and earlier calendar year of first primary diagnosis and shorter follow-up period were associated with increased risk for developing second primary neoplasm. Conclusions: Considering the long latency required for carcinogenesis, excess risk of second primary diagnoses soon after the first cancer, argues against the hypothesis of first primary treatment as an initiator for the second cancer. A detection bias of meticulously followed cancer patients, early exposure to common risk factors or genetic susceptibility of certain subpopulations for both malignancies seem plausible.