It has been suggested that endolithic algae inside the skeleton of cold-water corals might have a symbiotic relationship with the coral host and would positively affect coral calcification. However, so far this hypothesis has not yet been further explored. This study investigated the effect of endolithic algae on the growth performance and skeletal density of the cold-water coral Desmophyllum dianthus at Fjord Comau, southern Chile. The fluorescent staining agent calcein was used to document coral growth by measuring the upward linear extension of septa for a period of one and a half years. Observations on skeletal density were recorded using x-ray computed tomography. The results of this study show a severe reduction of growth rates associated with the presence of endolithic algae. Infested individuals grew about half as fast as non-infested polyps with median value of 1.18μm/day compared to 2.76μm/day. Data on skeletal density revealed a similar – although not statistically significant – trend displaying mean values of 2.160g/cm³ compared to 2.294g/cm³, respectively. These results point towards a parasitic relationship between D. dianthus and its endolithic algae refuting the hypothesis of a mutually beneficial association. However, although this study appears to conclusively indicate a negative effect of the association of D. dianthus with endolithic algae, controversial evidence has been discovered regarding the mode of the relationship. Despite the decrease in growth performance, the coral host seems to benefit from a low transfer of metabolites from the endoliths to the coral tissue. Further research will be necessary to fully resolve the matter.