The catastrophic anti-phospholipid syndrome (CAPS) differs from the anti-phospholipid syndrome in its accelerated systemic involvement leading to multi-organic failure. In this study, the occurrence of malignancies in patients with CAPS was evaluated and the clinical findings of CAPS patients with and without malignancies were compared. We investigated the web site-based international registry of patients with CAPS for all cases in which both CAPS and underlying malignancies were present. The clinical characteristics of these cases were subsequently evaluated to establish common characteristics. The CAPS registry included information on a total of 262 cases. Twenty-three (9%) patients suffered from malignancies. In 78% of these patients, the malignancy itself or the treatment modalities instituted for the carcinoma was the precipitating factor of CAPS. Only 39% of CAPS patients with malignancies recovered in comparison to 58% of patients without malignancies (p = 0.07). Treatment modalities, however, did not differ significantly between these patients. Infections were not evident as precipitating factors for any of the malignancy patients. The mean age of patients with malignancies was 9 years older than the average age of other patients with CAPS and the prevalence of SLE was significantly less common than in patients without malignancy. Malignancy may play a pathogenic role in patients with CAPS, whereas infections are more important as triggering factors in patients without malignancies. CAPS patients with malignancies are generally older than CAPS patients without malignancies; they generally have the worst prognosis of the entire CAPS cohort.