Purpose: To study the relationship between frequency of pain assessment and nursing workload, and also to analyze the frequency of pain assessment and its relation to knowledge and attitudes toward pain on nursing professionals in intensive care unit. Methods: An ambispective study was conducted in a Spanish tertiary-level intensive care unit between October 2017 and April 2018. For measurement of workload, the Nursing Activities Score scale was used, and for measurement of pain knowledge, the Knowledge and Attitudes Survey Regarding Pain was used. Results: There were 1,207 measurements among 41 nurses and 1,838 among 317 patients. The average nursing workload was high (70.97 points). We found statistically significant positive association between nursing workload and the frequency of assessment (p < .001), as well as frequency of assessment and patients with communicative capacity (p = .008). Conclusions: Nursing workload affects the registration and assessment of patients' pain, resulting in a greater number of records as the workload performed by nurses increases. It is necessary to study in greater depth how the severity of pain, gender of the patients, and workload of nurses influence pain registration and assessment. (c) 2020 American Society for Pain Management Nursing. Published by Elsevier Inc. All rights reserved.