Atypical psychosis with a periodic course of exacerbation and features of major psychiatric disorders [schizophrenia (SZ) and bipolar disorder (BD)] has a long history in clinical psychiatry in Japan. Based upon the new criteria of atypical psychosis, a Genome-Wide Association Study (GWAS) was conducted to identify the risk gene or variants. The relationships between atypical psychosis, SZ and BD were then assessed using independent GWAS data. Forty-seven patients with solid criteria of atypical psychosis and 882 normal controls (NCs) were scanned using an Affymetrics 6.0 chip. GWAS SZ data (560 SZ cases and 548 NCs) and GWAS BD (107 cases with BD type 1 and 107 NCs) were compared using gene-based analysis. The most significant SNPs were detected around the CHN2/CPVL genes (rs245914, P=1.6x10(-7)), COL21A1 gene (rs12196860, P=2.45x10(-7)), and PYGL/TRIM9 genes (rs1959536, P=7.73x10(-7)), although none of the single-nucleotide polymorphisms exhibited genome-wide significance (P=5x10(-8)). One of the highest peaks was detected on the major histocompatibility complex region, where large SZ GWASs have previously disclosed an association. The gene-based analysis suggested significant enrichment between SZ and atypical psychosis (P=0.01), but not BD. This study provides clues about the types of patient whose diagnosis lies between SZ and BD. Studies with larger samples are required to determine the causal variant. (c) 2013 Wiley Periodicals, Inc.