Although many individuals with obsessive compulsive disorder (OCD) show significant improvement after treatment with serotonin reuptake inhibitors (SRIs), the treatment effect is usually partial and residual symptoms remain in most cases despite continued treatment. The proportion of patients failing to achieve a satisfactory outcome is difficult to define, but may be estimated at approximately 40% of cases. Even after switching to a second SRI, approximately 30% of cases do not respond (March et al., 1997). While there is a wealth of empirical data supporting first-line treatment of OCD, the evidence–base for second-line treatments is slim. In this paper we review current evidence for co-administration of antipsychotics in SRI resistant cases of OCD, based upon, wherever possible, randomized controlled trials (RCTs). Uncontrolled studies are cited where systematic data are lacking (so far there have been no meta-analyses of treatment studies for this subgroup). A systematic search of electronic databases (EMBase [1974-date], MEDLINE [1966-date], PsychInfo [1987-date]) was run using combinations of the terms obsessive compulsive and (randomized or control or clinical trial or placebo or blind) or (systematic or review or meta-analysis), as well as individual drug names. This was complemented by consulting with colleagues in the field and reviewing recent unpublished data presented at international, peer-reviewed symposia. Most published studies have investigated acute treatment of OCD, with a shortage of data relating to the treatment-resistant condition. Previous reviews of this area (e.g., Sareen et al., 2004) have been superceded by the publication of several new studies within the last 12 months.