This parallel group randomized controlled trial tested effectiveness of the Let's Talk About Pain training on respite workers' (RW) pain-related knowledge and feasibility-confidence-skill ratings using between-group and within-group analyses. Fourteen children's respite organizations were randomized to pain or control training. Participants (n[subscript intervention] = 66; n[subscript control] = 92) underwent a 3--3.5 hour training and completed measures at pre-, post-, and 4--6 week follow-up. Intention-to-treat (n[subscript intervention] = 65; n[subscript control] = 92) and per-protocol (n[subscript intervention] = 26--38; n[subscript control] = 40--57) analyses were conducted. Pain training participants demonstrated significantly higher pain knowledge and feasibility-confidence-skill ratings post-follow-up versus control group and significant increases in knowledge from pre-post. Significant gains were maintained from post-follow-up. Results represent a promising step towards enhancing pain-related care for children with IDD. [For Part Two, see EJ1304421.]