Attention-deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder with high heritability. At least 30% of patients diagnosed in childhood continue to suffer ADHD during adulthood and genetic risk factors may play an essential role in the persistence of the disorder throughout lifespan. To date, Genome-Wide Association Studies (GWAS) of ADHD have been completed in seven independent datasets, six of which were pediatric samples and one on persistent ADHD using a DNA-pooling strategy, but none of them reported genome-wide significant associations. In an attempt to unravel novel genes for the persistence of ADHD into adulthood, we conducted the first two-stage GWAS in adults with ADHD. The discovery sample included 607 ADHD cases and 584 controls. Top signals were subsequently tested for replication in three independent follow-up samples of 2,104 ADHD patients and 1,901 controls. None of the findings exceeded the genome-wide threshold for significance (PGC<5e-08), but we found evidence for the involvement of the FBXO33 gene in combined ADHD in the discovery sample (P=9.02e-07) and in the joint analysis of both stages (P=9.7e-03). Additional evidence for a FBXO33 role in ADHD was found through gene-wise and pathway enrichment analyses in our genomic study. Risk alleles were associated with lower FBXO33 expression in lymphoblastoid cell lines and with reduced frontal grey matter volume in a sample of 1,300 adult subjects. Our findings point for the first time at the ubiquitination machinery as a new disease mechanism for adult ADHD and establish a rationale for searching for additional risk variants in ubiquitination-related genes.Neuropsychopharmacology accepted article preview online, 06 October 2014. doi:10.1038/npp.2014.267.