Data sources Six databases (MEDLINE, EMBASE, CENTRAL, ProQuest, Scopus and Web of Science) and two trial registries (ICTRP, ClinicalTrials.gov) were searched from their inception to May 2019 using a search strategy developed for OVID (MEDLINE/EMBASE).Study selection This systematic review included observational studies and randomised controlled trials related to the incidence of dental caries in patients with a history of head and neck radiotherapy. In addition, studies in which patients received radiotherapy treatment combined with surgical and/or chemotherapy treatment were also included. Study patients all had their teeth assessed and treated before radiotherapy. The diagnosis of caries may have been clinical and/or radiographic and, patients were followed for at least three months. Only English-language papers were included. Studies involving patients with recurrent cancer or with a repeated course of radiotherapy were excluded.Data extraction and synthesis Studies were selected by two independent reviewers, however, if a consensus could not be reached, a third reviewer was consulted. A total of 847 papers were selected through the search. Only 22 studies met the inclusion criteria. Selected articles included three randomised or quasi-randomised controlled trials and 19 observational studies, of which, seven were retrospective studies. The proportions of patients who developed caries after radiotherapy were pooled using a random-effects model. To inquire the between-study heterogeneity, a chi-square standard test was performed. To determine the degree of heterogeneity in proportions across studies, the I2 statistic was calculated. A funnel plot was built to evaluate the publication bias. To verify the correlation between relevant variables (radiation dose, chemotherapy in addition to radiotherapy, cancer site) and the proportion who developed dental caries, a meta-regression was performed. The STATA version 14 software was used for all statistical analysis.Results In the meta-analysis, the pooled percentage obtained of patients with head and neck cancer who developed dental caries post-radiotherapy, considering 15 included studies, was 29%. However, after excluding six studies on the basis of follow-up greater than two years, the pooled percentage increased to 37%. Furthermore, studies with a higher proportion of patients treated with chemotherapy in addition to radiotherapy (after the exclusion of an outlier), and also studies with a higher mean/median radiotherapy dose exposure, had an increased incidence of dental caries (p = 0.02). However, the results should be considered with caution due to the heterogeneity of the included studies.Conclusions The review showed that there is an incidence of approximately 29% of dental caries in patients with head and neck cancer after radiotherapy and, if considering only patients within 2 years post-radiotherapy, the percentage increases to approximately 37%. However, further studies are needed to allow a better understanding of the various factors related to the development of dental caries in patients with head and neck cancer treated with radiotherapy.