Background: Animal studies and uncontrolled case series in humans have suggested a possible association between breast implant exposure and monoclonal gammopathy. Objective: To assess whether there is an increased risk of monoclonal gammopathy in women with silicone breast implants, we conducted a retrospective study of women exposed to breast implants and matched nonexposed women nested within a prospective cohort study (the Nurses' Health Study). Methods: We used serum protein electrophoresis and immunoglobulin subtype by immunofixation to test 288 women exposed to breast implants and 288 age-matched, nonexposed women who previously had provided a blood sample (1989-1990) for monoclonal proteins. Results: Among the women exposed to breast implants, 5 had monoclonal gammopathy of undetermiMed significance (MGUS) compared with 4 women among those not exposed (odds ratio, 1.25; 95% confidence interval, 0.27-6.39). The distribution of isotypes was similar across exposure groups. The exposed women with MGUS tended to be older than the nonexposed women (mean age, 60.4 years vs 52.5 years, respectively; P=.03). None of the 9 women with MGUS had reported multiple myeloma or other hematologic malignancies up through 1996. Conclusions: We find little evidence to support a substantial increased risk of MGUS in women exposed to breast implants. Larger studies are needed to determine if a more modest relationship exists.