eTable 1.

Baseline characteristics of all trial participants by randomization group (n=16608)

	Combined hormone therapy (n=8506)		Placebo (n=8102)
	N	%	N	%	P-Value
Age group at screening					0.79
50-59	2837	33.4	2683	33.1
60-69	3854	45.3	3655	45.1
70-79	1815	21.3	1764	21.8
Race/ethnicity					0.31
White	7141	84.0	6805	84.0
Black	548	6.4	574	7.1
Hispanic	471	5.5	415	5.1
American Indian	25	0.3	30	0.4
Asian/Pacific Islander	194	2.3	169	2.1
Unknown	127	1.5	109	1.3
Education					0.19
0-8 years	202	2.4	177	2.2
Some high school	373	4.4	362	4.5
High school diploma/GED	1615	19.1	1609	20.0
School after high school	3357	39.7	3060	38.0
College degree or higher	2915	34.4	2839	35.3
Gail 5year risk					0.76
< 1.25	2806	33.0	2717	33.5
1.25 - < 1.75	2859	33.6	2703	33.4
≥ 1.75	2841	33.4	2682	33.1
Age at menarche					0.83
≤11	1725	20.3	1670	20.7
12-13	4578	54.0	4334	53.7
≥ 14	2182	25.7	2061	25.6
Years since menopause					0.49
< 5 yrs	1313	17.1	1225	16.3
5 - <10 yrs	1467	19.1	1486	19.8
10 - <15 yrs	1613	21.0	1567	20.9
≥ 15 yrs	3286	42.8	3230	43.0
Number of term pregnancies					0.38
Never pregnant/Never had term pregnancy	860	10.2	833	10.3
1	690	8.1	661	8.2
2	1908	22.5	1708	21.2
3	2020	23.9	1952	24.2
4	1416	16.7	1412	17.5
5+	1575	18.6	1500	18.6
Age at first birth, y (categories)					0.18
Never pregnant/No term pregnant	860	11.2	833	11.5
< 20	1124	14.6	1117	15.4
20 - 29	4996	64.8	4698	64.6
30+	727	9.4	624	8.6
Number of months breastfed					0.82
Never brstfd	3813	45.3	3669	45.8
brstfd ≤ 1 year	3150	37.5	2971	37.1
brstfd > 1 year	1446	17.2	1366	17.1
Oral contraceptive use ever	3695	43.4	3447	42.5	0.24
Oral contraceptive duration					0.25
< 5 yrs	1982	53.7	1781	51.7
5 - < 10 yrs	825	22.3	808	23.5
≥ 10 yrs	886	24.0	855	24.8
HRT use status					0.45
Never used	6277	73.8	6022	74.4
Past user	1671	19.7	1587	19.6
Current user	554	6.5	490	6.1
Unopposed estrogen use ever	903	10.6	865	10.7	0.90
Unopposed estrogen use					0.81
Non-user	7603	89.4	7237	89.3
< 5 yrs	677	8.0	659	8.1
≥ 5 yrs	226	2.7	205	2.5
Estrogen + progesterone use ever	1516	17.8	1396	17.2	0.32
Estrogen + Progest Duration					0.27
Non-user	6990	82.2	6706	82.8
< 5 yrs	1050	12.3	997	12.3
≥ 5 yrs	466	5.5	399	4.9
Time since quitting HT					0.58
Current	554	6.5	490	6.1
Past, < 5 yrs	726	8.5	673	8.3
Past, ≥ 5 yrs	945	11.1	914	11.3
Never	6277	73.8	6022	74.4
Number of first deg female relatives with breast cancer					0.23
None	6954	87.3	6676	88.2
1	927	11.6	816	10.8
2 or more	82	1.0	79	1.0
Benign breast disease					0.86
No	6340	83.6	6278	83.3
Yes, 1 biopsy	956	12.6	972	12.9
Yes, 2+ biopsies	290	3.8	288	3.8
Body-mass index (kg/m2), baseline					0.89
< 25	2579	30.4	2479	30.8
25 - < 30	2992	35.3	2835	35.2
≥ 30	2899	34.2	2737	34.0
Dietary energy(kcal)					0.40
≤ 1322 kcal	2687	32.7	2610	33.3
1322 < - 1841 kcal	2779	33.8	2678	34.2
> 1841 kcal	2752	33.5	2545	32.5
Percent energy from fat					0.46
≤ 29.6 percent	2709	33.0	2588	33.0
29.6 < - 37.2 percent	2764	33.6	2694	34.4
> 37.2 percent	2745	33.4	2551	32.6
Physical act (METS/wk)					0.73
≤ 3.5 METS/wk	2658	34.6	2603	34.3
3.5 < - 12.8 METS/wk	2508	32.7	2467	32.5
> 12.8 METS/wk	2505	32.7	2526	33.3
Alcohol use					0.25
Non Drinker	3601	42.5	3471	43.0
≤ 1 drink/day	3821	45.1	3546	44.0
> 1 drink/day	1047	12.4	1048	13.0
Smoking status					0.85
Never	4178	49.6	3999	50.0
Past	3362	39.9	3157	39.5
Current	880	10.5	838	10.5
NSAIDs	2853	33.5	2767	34.2	0.41

Due to information missing for some variables, category denominators do not alw ays equal group total shown in column heading.

Gail risk score incorporates age, history of benign disease (atypia staus unknown in the Women's Health Initiative), age at menarche, age at first live birth, race/ethnicity, and numbers of mothers and sisters with breast cancer

NSAIDs = use of aspirin, ibuprofen, prescription NSAIDs, or the related analgesic, acetaminophen

Time from menopause was defined as previously described¹⁵, as the interval from the onset of menopause to first menopausal hormone therapy use or first use of study medication (hormone or placebo).