This study assessed the effect of bur or Er:YAG laser preparation on marginal and internal adaptation of conventional and extended fissure sealing (FS) with a 3-component etch-and-rinse (Optibond FL; OFL) and a 1-component self-etch (Scotchbond Universal; SB) adhesive system. Scanning electron microscope analysis was performed before and after thermocycling/occlusal load and additional optical coherence tomography evaluation was carried out for internal marginal assessment. Significant differences were observed between the groups (ANOVA, p<0.05). Laser-prepared and non-etched FS suffered from marginal degradation after fatigue. When enamel was etched with H3PO4 and independently of the adhesive system, laser technique was equally effective to bur-preparation with percentages of continuous margins ranging from 96 to 99%, being laser less invasive than bur preparation. This is clinically relevant in paediatric dentistry as minimally invasive FS can be performed with laser and adhesive systems used as fissure sealants. Nevertheless, enamel etch with phosphoric acid is still necessary.