Abstract Background: Synovial sarcoma is a rare cancer with peak incidence in the young adult period. Despite poor outcomes of this aggressive cancer, there is little epidemiologic research addressing its etiology. Methods: We collected birth characteristic data on synovial sarcoma cases born during 1978–2015 and diagnosed during 1988–2015 in California (n = 244), and 12,200 controls frequency-matched on year of birth. We also constructed a dataset of cancer cases in siblings of sarcoma subjects to assess familial risk. Results: In multivariable logistic regression analyses, synovial sarcoma was more frequent in Hispanics compared with non-Hispanic whites [OR, 1.48; 95% confidence interval (CI), 1.06–2.08]. Higher birth weight was a risk factor in Hispanics; each 500 g increase in birth weight was associated with a 22% increase in disease risk (OR, 1.22; 95% CI, 1.00–1.48). Also, a strong role for birth order was suggested, with highest risk for the first born (second child compared with first: OR, 0.61; 95% CI, 0.44–0.84; third or later compared with first: OR, 0.53; 95% CI, 0.36–0.77). Siblings of patients with synovial sarcoma did not display elevated cancer incidence, suggesting the low likelihood that strong familial predisposition alleles play a significant role in this disease. Conclusions: The associations with birth weight and birth order suggest that nutritional, developmental, and environmental factors may play a role in the etiology of synovial sarcoma. Impact: Further epidemiologic research on synovial sarcoma should evaluate epigenetic and developmental mechanisms and the formation of the archetypical t(X;18) translocation that defines this disease.