When butt-welding metal sheets with high power lasers the gap distance between the sheets determine the final quality of the seam. In a number of systematic laboratory experiments the critical gap distance that results in sound beads is identified. By grinding the edges of the sheets, a number of "reference" welds are made and compared to sheets with the edges shear cut. The gap distance is precisely controlled by inserting spacers between the sheets. In the tests the gap is set at 0.00, 0.02, 0.05, 0.08 and 0.10 mm. Mild steel (St 1203) with thickness? of 0.75 and 1.25 mm with and without zinc coating were analysed. A total of 120 welds are made at different welding speeds.As quality norm DIN 8563 is used to divide the welds into quality classes. Since this norm only deals with surface defects a number of welds are also x-ray photographed.According to DIN 8563 the welds have classes of either B, C or D where B is best. If a weld falls outside of these classes it is termed class XX.Of the weld combinations analysed 80 % are of class B and 17 % of class XX. Thus the welds are either good or bad. Of the bad welds identified it is evident that the gap distance plays a major role. 90 % of the bad welds have a gap distance larger than 0.05 mm. The results also show that 85 % of the bad welds are shear cut and only 15 % ground. Furthermore the results show that two third of the bad welds are zinc coated.X-ray pictures revealed that welding at "low" welding speeds (2 m/min. for 0.75 mm and 1 m/min. for 1.25 mm sheets) results in a characteristic "Christmas tree" structure, useless for production. Thus, these welding speeds should be avoided.