Background Whereas there are numerous studies on unintentional weight loss (UWL), these have been limited by small sample sizes, short or variable follow‐up, and focus on older patients. Although some case series have revealed that malignancies escaping early detection and uncovered subsequently are exceptional, reported follow-ups have been too short or unspecified and necropsies seldom made. Our objective was to examine the etiologies, characteristics, and long-term outcome of UWL in a large cohort of outpatients. Methods We prospectively enrolled patients referred to an outpatient diagnosis unit for evaluation of UWL as a dominant or isolated feature of disease. Eligible patients underwent a standard baseline evaluation with laboratory tests and chest X-ray. Patients without identifiable causes 6 months after presentation underwent a systematic follow-up lasting for 60 further months. Subjects aged ≥65 years without initially recognizable causes underwent an oral cavity examination, a videofluoroscopy or swallowing study, and a depression and cognitive assessment. Results Overall, 2677 patients (mean age, 64.4 [14.7] years; 51% males) were included. Predominant etiologies were digestive organic disorders (nonmalignant in 17% and malignant in 16%). Psychosocial disorders explained 16% of cases. Oral disorders were second to nonhematologic malignancies as cause of UWL in patients aged ≥65 years. Although 375 (14%) patients were initially diagnosed with unexplained UWL, malignancies were detected in only 19 (5%) within the first 28 months after referral. Diagnosis was established at autopsy in 14 cases. Conclusion This investigation provides new information on the relevance of follow-up in the long-term clinical outcome of patients with unexplained UWL and on the role of age on this entity. Although unexplained UWL seldom constitutes a short-term medical alert, malignancies may be undetectable until death. Therefore, these patients should be followed up regularly (eg yearly visits) for longer than reported periods, and autopsies pursued when facing unsolved deaths.