This analysis traces the genesis, evolution, and function of the hitherto abandoned illness category of nostalgia and demonstrates how compelling environmental imperatives can generate a nosological construct (diagnosis) whose etiology changes according to prevailing living conditions. I show that, to a large extent, the physical symptoms of nostalgia are a function of abrupt changes in social structures and that the explanation for the nostalgic experience is influenced by cultural values and by the prevailing medical and social attitudes in various chronological periods. Nostalgia is a culturally devised label that controls and treats deviant behavior but fades when environmental circumstances restabilize. Although no longer a valid category, nostalgia helps therapists understand how to deal with homesickness, separation, and loss, and facilitates insight and coping among immigrants, sojourners, and culturally alienated persons.