Higher self-reported physical activity is associated with lower breast cancer incidence and mortality. Objectively measured timed walking speed, predictive of longevity in older adults, has been associated with ambulatory physical activity in small studies but definitive assessment of the association is lacking. Participants were a subset of 14 719 postmenopausal women in the Women's Health Initiative study who, at entry, had 10 m, timed walking speed determined. After 12.4 years [mean (SD) (3.5)] follow-up, 762 invasive breast cancers were diagnosed in this group. In addition, 8162 of these women self-reported physical activity. Simple linear regression was used to examine the relationship between timed walking speed and self-reported physical activity. A Cox proportional hazard model was used to estimate age-adjusted hazard ratios and 95% confidence intervals for the association between timed walking speed and invasive breast cancer incidence. Although a linear regression model for self-reported physical activity [log metabolic equivalent task (MET) h/week] versus 10 m, timed walking speed had a statistically significant slope (coefficient=0.03, P<0.0001, correlation=0.20), the magnitude of the relationship was not clinically useful. Timed walking speed quintile was not associated with breast cancer incidence in age-adjusted or multivariant analyses (P for trend=0.60). Timed walking speed was not associated with self-reported physical activity in a clinically useful manner or with breast cancer incidence. Our findings do not support use of timed walking speed as an objective surrogate for self-reported physical activity.