In this paper, we describe results of a study on evaluation of intralingual machine translation. The study focuses on machine translations of medical texts into Plain German. The automatically simplified texts were compared with manually simplified texts (i.e., simplified by human experts) as well as with the underlying, unsimplified source texts. We analyse the quality of outputs from three models based on different criteria, such as correctness, readability, and syntactic complexity. We compare the outputs of the three models under analysis between each other, as well as with the existing human translations. The study revealed that system performance depends on the evaluation criteria used and that only one of the three models showed strong similarities to the human translations. Furthermore, we identified various types of errors in all three models. These included not only grammatical mistakes and misspellings, but also incorrect explanations of technical terms and false statements, which in turn led to serious content-related mistakes.