Although affect dysregulation is considered a core component of borderline personality disorder (BPD) and somatoform disorders (SoD), remarkably little research has focused on prevalence and nature of affect dysregulation in these disorders. BPD and SoD diagnoses were confirmed or ruled out in 472 psychiatric inpatients using clinical interviews. Three qualitatively different forms of affect dysregulation were identified: under-regulation, over-regulation of affect and combined under- and over-regulation of affect. BPD was associated with under-regulation of affect, and SoD was associated with over-regulation of affect. However, one in five patients with BPD also reported substantial over-regulation, and one in six patients with SoD reported clinically significant under-regulation, and the comorbid BPD and SoD group reported more frequently both over- and under-regulation than patients diagnosed with BPD or SoD alone or those with other psychiatric disorders.