Dyskerin is a pseudouridine (ψ) synthase involved in fundamental cellular processes including uridine modification in rRNA and small nuclear RNA and telomere stabilization. Dyskerin functions are altered in X-linked dyskeratosis congenita (X-DC) and cancer. Dyskerin's role in rRNA pseudouridylation has been suggested to underlie the alterations in mRNA translation described in cells lacking dyskerin function, although relevant direct evidences are currently lacking. Our purpose was to establish definitely whether defective dyskerin function might determine an intrinsic ribosomal defect leading to an altered synthetic activity. Therefore, ribosomes from dyskerin-depleted human cells were purified and 1) added to a controlled reticulocyte cell-free system devoid of ribosomes to study mRNA translation; 2) analyzed for protein contamination and composition by mass spectrometry, 3) analyzed for global pseudouridylation levels. Ribosomes purified from dyskerin-depleted cells showed altered translational fidelity and internal ribosome entry site (IRES)-mediated translation. These ribosomes displayed reduced uridine modification, whereas they were not different in terms of protein contamination or ribosomal protein composition with respect to ribosomes from matched control cells with full dyskerin activity. In conclusion, lack of dyskerin function in human cells induces a defect in rRNA uridine modification, which is sufficient to alter ribosome activity.