Word stem completion tasks involve showing participants a number of words and then later asking them to complete word stems to make a full word. If the stem is completed with one of the studied words, it indicates memory. It is a test widely used to assess both implicit and explicit forms of memory. An important aspect of stimulus selection is that target words should not frequently be generated spontaneously from the word stem, to ensure that production of the word really represents memory. In this article, we present a database of spontaneous stem completion rates for 395 stems from a group of 80 British undergraduate psychology students. It includes information on other characteristics of the words (word frequency, concreteness, imageability, age of acquisition, common part of speech, and number of letters) and, as such, can be used to select suitable words to include in a stem completion task. Supplemental materials for this article may be downloaded from http://brm .psychonomic-journals.org/content/supplemental.Word stem completion (WSC) tasks have a long history of use in psychology, where their initial introduction indicated that implicit memory can be preserved in amnesic patients (Warrington & Weiskrantz, 1970). Participants are shown lists of words, unaware of a later memory task. Subsequently, they are presented with the first few letters of a word and are asked to complete this stem with the first word that comes to mind (see Roediger, Weldon, Stadler, & Riegler, 1992, for a review). Explicit memory versions ask for completions that are remembered from the study list (Greene, 1986). An increase in stem completion levels due to preexposure of the words indicates memory. This methodology is still widely used in tasks ranging from cerebral blood flow measures (Sorond, Schnyer, Serrador, Milberg, & Lipsitz, 2008) to behavioral tasks in healthy participants (Benjamin Clarke & Butler, 2008) and patients (e.g., Alzheimer's disease, LaVoie & Faulkner, 2008; schizophrenia, Pope & Kern, 2006).The selection of words to use in WSC tasks is critical. The stem must be able to form multiple words when completed. Words should not be proper nouns and should have sufficient frequency for participants to know them and their meanings. They should have relatively matched imageability and concreteness. Information on these characteristics is available through established databases (e.g., M. Wilson, 1988).One key criterion, particularly important for implicit tasks, is that the words used should not commonly be produced in a spontaneous WSC task. Since participants must generate the first completion that comes to mind, a frequent spontaneous completion reduces the distance between chance and ceiling performance. In imaging studies, "correct" responses would be more likely to be guesses, adding unwanted noise to the neural signal. Matching low levels of spontaneous WSC across word lists-and ideally, all words-ensures that tasks are sensitive and reliable.Spontaneous WSC rates are not easily available, and studies have run p...