The present study provides data on the reliability, validity and treatment sensitivity of the Liebowitz Social Anxiety Scale (LSAS), one of the most commonly used clinician-administered scales for the assessment of social phobia. Three hundred and eighty-two patients from several studies of the treatment of social phobia were evaluated. An independent assessor administered the LSAS to each patient prior to the initiation of treatment. Patients also completed other measures of social anxiety and avoidance, although the specific measures varied across samples. The LSAS and its subscales were normally distributed and demonstrated excellent internal consistency. The convergent validity of the LSAS was demonstrated via significant correlations with other commonly-used measures of social anxiety and avoidance. These correlations also tended to be larger than correlations with measures of depression, especially after treatment. However, the pattern of correlations of LSAS subscales with one another and with the other measures suggest that the fear subscales and the avoidance subscales may not be sufficiently distinct in clinical samples. The LSAS was also demonstrated to be sensitive to the effects of pharmacological treatments of social phobia over time and in comparison to double-blind pill placebo. The LSAS appears to be a reliable, valid and treatment sensitive measure of social phobia. Further study of the LSAS, both in samples with severe social phobia and in community samples, is needed.