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A previously healthy man in his 60s being worked up for splenomegaly presented to the emergency department with recurrent episodes of angioedema. Each episode was attributed to a precipitating cause, and consequently, the predisposing C1 esterase inhibitor (C1-INH) deficiency remained undiagnosed until the third presentation. The aetiology of acquired C1-INH deficiency would be primarily obscure and require further investigations to identify. A clonal B cell population was finally isolated by flow cytometry after multiple repeat marrow samples, and a diagnosis of splenic marginal zone lymphoma was subsequently reached. Response to single-agent rituximab was observed with resolution of splenomegaly, disappearance of the antibody and restoration of C1-INH levels.