Concerns regarding the possible role of breast implants (particularly silicone breast implants) in the development of connective tissue diseases were raised by case reports of connective tissue diseases in women with breast implants. Case reports, however, are not appropriate for causation assessment. Within the past few years, epidemiologic studies have begun to appear. Based on a comprehensive literature search, 15 epidemiologic studies on breast implants and connective tissue diseases, which satisfied certain basic epidemiologic requirements, were included in the critical assessment. These studies utilized either the case-control or the cohort study design. Although each individual study was relatively small, and the statistical power to detect a modest risk increase in specific categories of connective tissue diseases was limited, the results of these studies, however, were strikingly consistent, particularly those reported in case-control studies. To increase statistical power and to take the consistency of results into consideration, meta-analyses were used to summarize results from individual studies quantitatively. Based on data from case-control studies, meta-analyses of rheumatoid arthritis, systemic sclerosis (scleroderma), and systemic lupus erythematosus were performed. These case-control studies represented a combined database of approximately 4000 cases of connective tissue diseases, and the power was sufficient to detect a relatively small increase in risk. Based on the meta-analyses, the relative risks (95% confidence intervals) were 0.85 (0.48-1.51) for rheumatoid arthritis, 0.82 (0. 50-1.35) for systemic sclerosis, and 0.33 (0.06-2.03) for systemic lupus erythematosus, indicating that there was no increased risk of connective tissue diseases associated with breast implants. The findings derived from the meta-analyses of case-control studies were supported by results from cohort or prospective studies. It was concluded that epidemiologic data did not provide any evidence for a causal relationship between silicone breast implants and connective tissue diseases.