A 51 year old male presented to a neurology outpatient clinic with symptoms of chronic muscle pain and weakness. He had been on long-term danazol for hereditary angioedema. Biochemical tests revealed hypogonadism, hypocortisolism with a borderline cortisol response to tetracosactrin stimulation, and subclinical hypothyroidism. The measured testosterone level was 2 mmol/L higher by immunoassay than by liquid chromatography-tandem mass spectrometry. Cessation of danazol for a four week period led to an increase in serum cortisol and testosterone and a decrease in thyroid stimulating hormone, but minimal improvement in muscle symptoms and an increased frequency of angioedema attacks. Positive interference by danazol in the testosterone immunoassay is reported by the manufacturer, which we replicated in a small experiment using danazol-spiked samples of stripped serum and pooled female plasma. As an androgenic steroid, it suppresses the secretion of gonadotropins and sex steroids. It also displaces cortisol from its major binding protein, corticosteroid-binding globulin (CBG), which decreases the measured total cortisol level but probably does not affect the free, active fraction. Danazol can also decrease thyroid binding globulin. Danazol has a number of real and artefactual effects on the laboratory results of testosterone, gonadotropins, cortisol and thyroid function. Where an apparent reduction of testosterone and cortisol occur with danazol therapy, it is unclear whether hormone replacement is warranted.