The synthesis of pure silica MWW type zeolite ITQ-1 using trimethyladamantammonium (TMAda(+)) is described. The reproducibility of the synthesis, as well as the duality of the materials obtained, is greatly improved if the synthesis is assisted by hexamethyleneimine (HMI) as a second organic additive. As TMAda(+) is too large to fit into the sinusoidal 10 MR channel (i.e., the channel delimited by a ring of 10 tetrahedra), the stabilization of this channel seems to require the presence of additional organic moieties. In the absence of HMI, these probably come from either the partial degradation of TMAda(+) or from organics adsorbed on the PTFE liners in previous syntheses. The use of TMAda(+) and HMI and Na+ cations allows a fast and highly reproducible synthesis of pure silica ITQ-1. The material obtained shows an improved crystallinity in both the as-made and the calcined form. The structure of calcined ITQ-1 has been refined in space group P6/mmm (a = 14.2081(1) Angstrom, c = 24.945(2) Angstrom, R-exp = 0.103, R-wp = 0.159, R-f = 0.065), using synchrotron powder diffraction data, and the topology previously proposed for the aluminosilicate MCM-22 zeolite has been confirmed. Comparison of the highly resolved Si-29 MAS NMR spectra of as-made and calcined ITQ-1 show that in the as-made form there is a high concentration of Si-OH defect groups, which are annealed upon calcination and which arise from a lack of connectivity between specific Si sites.