Quinolinic acid phosphoribosyltransferase (NadC; EC 2.4.2.19) is the key enzyme of NAD(+) biosynthesis in both prokaryotes and eukaryotes. NadC catalyzes the decarboxylation of quinolinic acid (QA) to produce nicotinic acid mononucleotide (NAMN), an intermediate in NAD synthesis. NadCs of Helicobacter pylori appeared to be a hexamer during the purification procedure. Three different complexes of NadC, with QA, NAMN and phthalic acid (PA), an analogue of QA, were crystallized at 294 +/- 1 K using the hanging-drop vapour-diffusion method. The QA complex crystal was found to belong to space group P4(1)2(1)2, with unit-cell parameters a = b = 148.8, c = 145.7 A, alpha = beta = gamma = 90 degrees. Diffraction data were collected from the NadC-substrate and NadC-substrate analogue complexes to resolutions of 2.3 A (QA), 2.8 A (PA) and 3.3 A (NAMN) using synchrotron X-ray radiation.