Summary Hydroxyproline excretion was studied in 17 patients with classical Marfan's syndrome from 11 to 33 years of age and compared with age-matched controls. Pre-adolescent and adult Marfan's consistently excreted greater amounts of hydroxyproline than controls. For the 13- to 18-year-old group, the variation was too great to show definite differences. The authors wish to thank Dr. Victor A. McKusick and staff, Johns Hopkins Hospital, Baltimore, Md., for encouraging this work and making available several patients with Marfan's syndrome. We appreciate the help given by the staff at Flower-Fifth Avenue Hospital, Metropolitan Hospital, and St. Luke's Hospital, New York, in assisting in collecting the urines.