The core study is a multi-center, randomized, double-masked, placebo-controlled clinical trial enrolling a total of 300 non-smoking persons with emphysema at 5 participating clinical centers. Study participants were randomly assigned in a 1:1:1 ratio to one of three retinoid treatment arms. Within each study arm, participants were randomized to active treatment or to a matched placebo in a 3:1 ratio. After six months of follow-up, the three month crossover period began. Participants initially assigned to active treatment began taking the matched placebo, and participants originally assigned placebo began taking the active treatment. Retinoid treatment arms were all-trans retinoic acid (ATRA trademark, Roche Laboratories, Inc.) at two different doses and 13-cis retinoic acid (cRA, Roche Laboratories, Inc.), each with matched placebo. Several single-center substudies were also performed.