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The table below summarizes the available evidence on the risks and benefits of hormone therapy. Included are all randomized trials and meta-analyses looking at disease incidence or mortality. Inclusion of observational trials was dependent on size: case-control studies with 500 or more cases of the disease and cohort studies with at least 2,000 women are included here.

The year noted is when the study results were published. The name following the title of the study is the last name of the first author on the publication.

E = estrogen; P = progestin

Year	Study	Study Type	Participants	Hormones	Results	Other Notes
1998	Heart and Estrogen/progestin Replacement Study (HERS), Hulley	Randomized	2,763	E + P	39% increase in biliary tract surgery	Average follow-up 4.1 years
2000	Mamdani	Cohort	800,000	E, E + P	90% increase in gall bladder surgery	No distinction between E and E + P
2002	Heart and Estrogen/progestin Replacement Study (HERS) II, Hulley	Randomized	2,321	E + P	44% increase in billiary tract surgery	2.7 year extension of HERS with subset of participants from original study; 6.8 years total follow-up