Complement factor H (β‐1H globulin) is an important regulatory protein which inhibits the spontaneous complement activation via the alternative pathway. We describe a 15‐year‐old girl without any detectable factor H in plasma. She has had two episodes of meningococcal disease, but is otherwise completely healthy. Secondary to the factor‐H deficiency, the levels of factor B, properdin, C3, and C5‐C9 were strongly reduced due to spontaneous in vivo activation of the alternative complement pathway. Plasma C3dg was strongly elevated in spite of the Factor‐H deficiency; apparently erythrocyte CR1 substitutes for factor H in C3 degradation. Neither C3 nor complement lesions were demonstrable on her erythrocytes which did, however, show increased, spontaneous haemolysis in vitro in citrate plasma, but not in serum. The patient is a single child and her parents, who are unrelated and healthy, had half‐normal levels of factor H. This reduction of factor H is sufficient to cause increased, spontaneous activation of the alternative pathway.