The ability of a tetralin phenoxy isobutyrate, SU‐13437, to lower serum lipids was compared with that of a matching placebo in a double‐blind crossover study. Treatment with SU‐13437 for 4 weeks lowered serum cholesterol by 23 per cent in hypercholesterolemic patients, and it lowered serum triglycerides by 68 per cent in hypertriglyceridemic patients. None of the patients failed to respond to treatment. Gastrointestinal upset was seen in 2 of the 29 patients, and one patient had transient elevation of serum glutamic oxaloacetic transaminase.