PURPOSE: To evaluate how laboratory polishing and glazing and clinical adjustments, i.e., by grinding and polishing affect the biaxial flexural strength and surface characterization of multilayer zirconia. METHODS: Specimens of KATANA™ Zirconia YML, containing Enamel-Body 1 or Body 2-3 layers, and UTML were divided into 18 groups (n = 11) according to material/layer and finish treatment: laboratory polishing, glazing, or both, or followed by clinical adjustments. The surface roughness, surface structure and elemental composition were evaluated. Thermocyclic-mechanical cyclic loading and biaxial flexural strength test were performed. Three- and two-way ANOVA were used (α = 0.05). RESULTS: The yttrium amount decreased in the order UTML, YML Enamel-Body 1, and YML Body 2-3. Body 2-3 showed higher flexural strength than Enamel-Body 1 and UTML, in that order. Flexural strength after clinical adjustments was higher in Body 2-3 but lower in Enamel-Body 1 and UTML. Finish treatments had indistinguishable impact on Enamel-Body 1. Body 2-3 had higher strength after polishing, polishing and glazing, and glazing, in that order. UTML displayed the highest strength after polishing and glazing or glazing. The surface roughness was lowest after polishing regardless of material/layer. CONCLUSIONS: Laboratory treatments and clinical adjustments of multilayer zirconia affect the flexural strength and surface roughness differently depending on the zirconia type. Composition-gradient zirconia should be polished, and shade-gradient zirconia (≥ 5 mol% yttria) should be polished and glazed. Clinical adjustments of zirconia with higher yttria content are detrimental for the flexural strength and should be performed with caution.