Substantial cross-sectional correlations have been reported between the GHQ and PSE (and CIS) total scores. Although necessary, this is not a sufficient condition for assuming good validity of the GHQ as a severity measure in longitudinal and health care evaluation studies. For this purpose the GHQ should also accurately reflect changes in severity over time. To examine their concurrent validity, GHQ and PSE scores were compared, in a three-wave longitudinal study, among 175 new psychiatric out-patients. Using a longitudinal structural equation model that takes measurement error into account, the strength of both the cross-sectional and longitudinal relationship between GHQ and PSE were estimated. The GHQ performed remarkably well; changes in severity as defined by PSE-ID and PSE total score were clearly reflected by changes in GHQ scores. The revised scoring method of the GHQ proposed by Goodchild and Duncan-Jones did not yield superior results.