Background <p>There is limited evidence on whether the implementation of clinical guidelines has a positive effect on the symptomatology of cancer patients in palliative care. A large national project conducted 2017-2019 aimed to improve the quality of life of cancer patients by implementing clinical guidelines for severe pain, dyspnea, constipation and depression in Danish specialist palliative care. We investigated how pain, dyspnea, constipation and depression changed from the start of palliative care to 1-4 weeks later and whether changes were associated with timing of guideline implementation and with receipt of treatment according to guidelines.</p> Methods <p>A prospective, national quality improvement study. Data for the project was obtained from the Danish Palliative Care Database. We included adult patients with advanced cancer who answered the European Organisation for Research and Treatment of Cancer Quality of Life Questionnire-Core-15-Palliative Care (EORTC QLQ-C15-PAL) questionnaire at the start of specialist palliative care and 1–4 weeks later. Multiple linear regression evaluated associations between changes in symptom severity and timing of guideline implementation and receiving treatment according to guidelines.</p> Results <p>Of 4,640 patients completing both EORTC QLQ-C15-PAL questionnaires, 3,879 (84%) qualified for one or more treatment guidelines. Pain, dyspnea, constipation and depression improved significantly. Timing of guideline implementation and receiving treatment according to guidelines were not associated with improvement, except for dyspnea, where small associations were observed.</p> Conclusions <p>Patients reported improved pain, dyspnea, constipation and depression, but this improvement was not consistently associated with the implementation of the four guidelines in palliative care. The lack of impact may be attributed to a modest impact on clinical practice or the observational project design.</p>