Background: Health systems with limited resources may have the greatest impact on suicide if their prevention efforts target the highest-risk treatment groups during the highest-risk periods. To date, few health systems have carefully segmented their depression treatment Populations by level of risk and prioritized prevention efforts on this basis. Methods: We conducted a retrospective cohort study of 887,859 VA patients receiving depression treatment between 4/1/1999 and 9/30/2004. We calculated suicide rates for five sequential 12-week periods following treatment events that health systems could readily identify: psychiatric hospitalizations, new antidepressant starts (> 6 months without fills), “other” antidepressant starts, and dose changes. Using piecewise exponential models, we examined whether rates differed across time-periods. We also examined whether Suicide rates differed by age-group in these periods. Results: Over all time-periods, the suicide rate was 114/100,000 person-years (95% CI; 108, 120). In the first 12-week periods, suicide rates were: 568/100,000 p-y (95% CI; 493, 651) following psychiatric hospitalizations; 210/100,000 p-y (95% CI; 187, 236) following new antidepressant starts; 193/100,000 p-y (95% CI; 167, 222) following other starts; and 154/100,000 p-y (95% CI; 133, 177) following dose changes. Suicide rates remained above the base rate for 48 weeks following hospital discharge and 12 weeks following antidepressant events. Adults aged 61-80 years were at highest risk in the first 12-week periods. Conclusions: To have the greatest impact on Suicide, health systems should prioritize prevention efforts following psychiatric hospitalizations. If resources allow, closer monitoring may also be warranted in the first 12 weeks following antidepressant starts, across all age-groups. Published by Elsevier B.V.