Context: Exposure to traumatic events should be systematically assessed in health care services so that trauma-related treatment can be offered when appropriate. However, professionals often lack expertise in trauma inquiry and response, and therefore require training in this field. Objective: We aimed to determine whether the “Learning how to ask” training for trauma inquiry and response (Read, Hammersley, & Rudegeair, 2007) is effective in increasing health care professionals’ trauma inquiry behavior. Method: 148 professionals working in outpatient substance use disorder (SUD) services were cluster-randomized into an intervention or a waiting control group. The intervention group received a 1-day training and a refresher session 3 months later, while the control group received no training. The primary outcome was the change from baseline in the frequency of asking clients about traumatic events. Secondary outcomes were professionals’ evaluation of the training, knowledge, attitudes toward and confidence in trauma inquiry and response. Results: Change from baseline in the frequency of asking clients about traumatic events was significantly greater at 3-month and 6-month follow-up in the intervention group compared with the control group (b = 0.43, 95% CI [0.27, 0.59], p < .001). The training was positively evaluated by the participants. Knowledge, positive attitudes toward and confidence in trauma inquiry and response all showed significantly greater increases in the intervention group than in the control group. Conclusions: These findings suggest that health care professionals can acquire skills in trauma inquiry and response from short trainings, which may enhance systematic assessment of traumatic events.