Especially in recognition of spontaneous speech it is necessary to be able to cope with the occurance of unknown words. We present an approach to unknown word detection integrated with speech recognition. Though several schemes exist for the evaluation of detection algorithms all suffer from some deficiencies. Therefore, we also present a new evaluation measure called detection accuracy which is similar to the widely accepted word accuracy. We applied this measure to the evaluation of our approach on a large spontaneous speech recognition task from the German Verbmobil project.