Background: Informal cancer caregivers provide essential support to cancer patients, including performing direct medical/nursing tasks, assisting with activities of daily living, and offering social support. This study examined associations between the receipt of medical/nursing skills training and the caregiver burden as well as the mediation of caregiving confidence on this relationship in a sample of caregivers of lung and colorectal cancer patients.; Methods: Caregivers who had been identified by cancer patients in the Cancer Care Outcomes Research and Surveillance consortium completed a questionnaire assessing the care provided, the type of medical/nursing skills training received, the burden (measured with the modified short-form Zarit Burden Interview), and the confidence in caring for their patient's physical needs. Regression models that had been adjusted for sociodemographic, caregiver, and care recipient characteristics assessed the relationship between training received and burden, and a mediation analysis assessed the role of confidence in this relationship.; Results: Six hundred forty-one caregivers performed some type of medical/nursing task, with 59% (n = 377) reporting that they did not receive training for all the care provided. Caregivers reported moderate levels of burden (mean summary score, 32.07; standard deviation, 12.66; possible range, 14-70), and a lack of receipt of training was associated with greater levels of burden (b = 2.60; standard error, 0.98; P = .01). Confidence partially mediated the relation between training and burden (Sobel's t = 1.90; P = 0.03).; Conclusions: As the number of cancer patients and caregivers increases, understanding how best to reduce the caregiver burden is necessary. Skills training is a potential area for interventions, but research on how best to provide training for caregivers (ie, the content, mode of delivery, and timing) is needed.