A comprehensive equipment system is described that induces, collects, and analyzes sweat as a diagnostic procedure for cystic fibrosis. The safety and convenience of the iontophoretic induction phase have been improved by specially designed circuitry and electrodes. The problem of potentially serious error as a result of condensation within sweat-collection cups has been eliminated by use of a thermostatically controlled heated cup. The osmolality of the collected sweat is measured with a vapor-pressure osmometer. Data are presented to support the validity of osmolality as an alternative to electrolyte assay or electrical conductivity measurement and normal and abnormal ranges are provided. The complete system was tested in a pediatric hospital and data are presented to illustrate the relative advantages of the system with respect to safety, convenience, diagnostic reliability, and decreased potential for human error.