The phosphodiesterase 4B (PDE4B), which is involved in cognitive function in animal models, is a candidate susceptibility gene for schizophrenia (SZ) and bipolar disorder (BP). Variations in PDE4B have previously been associated with SZ, with a suggested gender-specific effect. We have genotyped and analyzed 40 and 72 tagging single nucleotide polymorphisms (tagSNPs) in SZ and BP multicenter samples, respectively, from the Scandinavian Collaboration on Psychiatric Etiology (SCOPE), involving 837 SZ cases and 1,473 controls plus 594 BP cases and 1,421 partly overlapping controls. Six and 16 tagSNPs were nominally associated (0.0005