Aims: To establish whether or not the state of patient oral health can influence the occurrence and/or severity of oral mucositis during hematopoietic progenitor cell transplantation (HPCT). Materials and Methods: The study included 72 patients awaiting HPCT. Prior to transplantation, clinical explora-exxploration and radiology were carried out and oral photographs were taken. This evaluated the exxtent of caries present, the number of missing teeth and the number of dental fillings in each patient; CAO (Caries and Obturations Index) DMFS (Decayed, Missing, and Filled Surfaces) and Restoration Indices were calculated. Gingival pathology was also examined by means of the Ainamo and Bay Gingival Bleeding Index. O'Leary's Plaque Index was used to evaluate the level of patient oral hygiene. This data was analyzed to see if it exercised any influence on the mucositis grade suffered during HPCT. Results: 96,87% of patients suffered some degree of mucositis during their treatment by the Transplant Unit. The grade of mucositis was seen to be influenced by the number of missing teeth (ANOVA p<0.016) and by the DMFS Index (ANOVA p< 0.038). Although this was not one of the aims of this study, patient age and the administration of colony-stimulating factors were also seen to influence these clinical manifestations. Conclusions: The state of prior oral health can influence decisively the mucositis suffered during transplantation. © Medicina Oral S. L.