Although the precise etiologic nature of obsessive-compulsive disorder (OCD), one of the most common psychiatric conditions, is unknown, several findings indicate involvement of the serotonin (5-HT) transporter. Apart from the specific effects of selective 5-HT reuptake inhibitors, other studies show decreased functionality of the platelet 5-HT transporter in OCD. In this report, the authors combine data from two independent studies of patients with OCD, showing both an increased activity of protein kinase type C (PKC) and a decreased activity of protein kinase type A (PKA). The authors propose a unifying hypothesis that OCD might be determined by an imbalance between PKC and PKA, with a prevalence of the former and, more generally, of the phosphoinositide over the cyclic adenosine monophosphate (cAMP) pathway. Should this hypothesis prove correct, the path would be open for new therapeutic interventions in the treatment of OCD.