OBJECTIVES: Our goal was to determine the risk of cancer after hospitalization for endometriosis.

STUDY DESIGN: Records of 20,686 women hospitalized with endometriosis during the period 1969 to 1983, as identified through the nationwide Swedish Inpatient Register, were linked against the National Swedish Cancer Registry through 1989 to identify all subsequent diagnoses of cancer. The study subjects were followed up for a mean of 11.4 years, with the cohort contributing 216,851 woman years of follow-up. Standardized incidence ratios were computed by the use of age- and period-specific incidence rates derived from the Swedish population. Because of the high proportion of subjects with gynecologic operations (55.6%), evaluation of the risk of gynecologic cancers involved truncation of person years at the time of any such operation.

RESULTS: The overall cancer risk was 1.2 (95% confidence interval 1.1 to 1.3). Significant excesses were observed for breast cancer (standardized incidence ratio = 1.3, 95% confidence interval 1.1 to 1.4), ovarian cancer (1.9, 1.3 to 2.8), and hematopoietic malignancies (1.4, 1.0 to 1.8); this latter excess was largely driven by an excess risk of non-Hodgkin's lymphoma (1.8, 1.2 to 2.6). The risk of ovarian cancer was particularly elevated among subjects with a long-standing history of ovarian endometriosis (4.2, 2.0 to 7.7). Cervical cancer risk was slightly reduced (0.7, 0.4 to 1.3) whereas no association was observed for cancer of the endometrium (1.1, 0.6 to 1.9).

CONCLUSIONS: These findings suggest that further attention be given to the risk of breast, ovarian and hematopoietic cancers among women with endometriosis and to exploring possible hormonal and immunologic reasons for the excess risks. (Am J Obstet Gynecol 1997;176:572-9