Bariatric procedures are an effective option for weight loss and control of comorbidities in obese patients. Obesity is a proinflammatory condition in which some cytokines such as leptin, a proinflammatory protein, is elevated and adiponectin, an anti-inflammatory protein, is decreased. In patients undergoing weight reduction surgeries, these hormone levels behave paradoxically. It is not known whether bariatric surgery protects against development of autoinflammatory or autoimmune conditions; nevertheless, changes occurring in the immune system are incompletely understood. In this case series, we describe 4 patients undergoing bariatric surgery, who subsequently developed systemic autoimmune diseases. Patients in our case series were asymptomatic before surgery and developed an autoimmune disease within 11.2 months. Two women fulfilled criteria for systemic lupus erythematosus (one associated with antiphospholipid syndrome), and 2 men developed rheumatoid arthritis. A causal relationship is difficult to establish because factors that could trigger these diseases are multiple, including genetic susceptibility, time elapsed until achievement of ideal weight, and vitamin deficiencies, among others. However, clinicians must be attentive to this possible association.