To investigate the risks of connective tissue diseases (CTDs) following breast implants we used the nationwide Danish Hospital Discharge Register (HDR) to identify 2,570 women who received breast implants, either for cosmetic reasons (N = 1,135) or for breast reconstruction (N = 1,435), between 1977 and 1992. Two additional cohorts of women having either breast reduction surgery (N = 7,071) or breast cancer without implants (N = 3,952) were identified for comparison. Observed-to-expected (O/E) cases of CTDs and other rheumatic conditions were calculated based on national hospital discharge rates. The calculated O/E ratio for definite CTDs was 1.1 (95% confidence interval [CI], 0.2-3.4) among women with cosmetic breast implants, and 1.3 (95% CI, 0.5-2.6) among women receiving implants for breast reconstruction. No CTD excesses were seen in the breast reduction or breast-cancer-without-implant cohorts. Statistically significant risks for muscular rheumatism (a nonspecific discharge diagnosis) were observed in all four patient cohorts: cosmetic (O/E ratio, 2.5; 95% CI, 1.7-3.6), breast reconstruction (O/E ratio, 2.5; 95% CI, 1.7-3.4), breast reduction (O/E ratio, 2.0; 95% CI, 1.6-2.3), and breast cancer without implants (O/E ratio, 1.4; 95% CI, 1.0-1.9). In conclusion, breast implants showed little association with definite CTDs. Breast surgery per so, however, was associated with an apparent increase in muscular rheumatism.