Goal of this study was to investigate whether appropriately applied spa therapy in several indications could be associated with a subsequent fall in the need for costly health services and missed working days due to sick-leave. The Naiade project was a multicenter observational, longitudinal, questionnaire-based study comparing an "entry" inquiry addressed to patients before an entry thermal cycle, and a "return" inquiry after 1 year. Routine statistical methods were used for comparisons. The study was carried out in 297 of the 340 certified Italian spa centers. Inquiries were managed by the spa doctor(s), with the collaboration of family doctors, and when necessary, hospitals, other health services, labour offices and employers. After exclusion of regular customers and of patients with acute disease phases or severe health conditions, 39,943 patients divided into eight diseases subgroups (rheumatic, respiratory, dermatologic, gynaecologic, otorhynologic, urinary, vascular and gastroenteric) underwent entry inquiry and appropriate spa treatment. Patients who returned for treatment after 1 year ("index year") were 23,680 (59.2%) and received return inquiry. Outcomes considered were: frequency and duration of hospitalisation periods; missed working days; regular use of disease-specific drugs; and resort to "non-spa" rehabilitation therapies. The data collected at return inquiry were compared with those of entry inquiry. All the considered outcomes appeared to be significantly reduced in the index year in seven of the eight disease subgroups in comparison with the previous year. In conclusion, disease-appropriate spa treatments were followed by a reduction in the need of subsequent health interventions in most disease subgroups. The health promoting value of spa treatments should therefore undergo more rigorous assessment with randomised controlled studies.