Plain language summarization, or lay summarization, is an emerging natural language processing task, aiming to make scientific articles accessible to an audience of non-scientific backgrounds. The healthcare domain can greatly benefit from applications of automatic plain language summarization, as results that concern a large portion of the population are reported in large documents with complex terminology. However, existing corpora for this task are limited in scope, usually regarding conference or journal article abstracts. In this paper, we introduce the task of automated generation of plain language summaries for clinical trials, and construct CARES (Clinical Abstractive Result Extraction and Simplification), the first corresponding dataset. CARES consists of publicly available, human-written summaries of clinical trials conducted by Pfizer. Source text is identified from documents released throughout the life-cycle of the trial, and steps are taken to remove noise and select the appropriate sections. Experiments show that state-of-the-art models achieve satisfactory results in most evaluation metrics