Data are presented to support the reliability, validity, and utility of a severity metric for phonological disorders. The metric, percentage of consonants correct (PCC), is readily derived from a continuous speech sample. PCC values are shown to reflect an ordinal severity scale that embraces the constructs of disability, intelligibility, and handicap. PCC values index four levels of "severity of involvement": Mild, mild-moderate, moderate-severe, and severe. The metric provides a means by which instructors, researchers, and speech-language pathologists working in different settings can specify subject descriptions, gauge the effects of intervention programs, and undertake cross-institutional projects. The metric is used as one component of a diagnostic classification system for phonological disorders (Shriberg & Kwiatkowski, 1982a) and within a framework for management of persons with phonological disorders (Shriberg & Kwiatkowski, 1982b), the first and second papers in this series.