Science writing curricula often stress the importance of using explanations to make a story understandable to readers. This study examines the use of explanation in news coverage of the first stem cell controversy in the U.S., through a content analysis of 343 news stories appearing in three major newspapers, three national newsweekly magazines, and three national network television news websites from 1994 to 2001. Two-thirds of the explanations were simple definitions. Consistent with previous research, the use of explanation was highest in specialized science sections and did not vary according to story length. However, online sources had the highest proportion of stories with in-text explanation (excluding links to related materials), challenging the idea that science explanation usage is deadline dependent. Stem-cell stories that reflected more play, in terms of length and placement, did not include more scientific explanations. However, more scientific explanations did appear in science or health section stories about stem-cell research, regardless of story play. Traditional print newspapers and newsmagazines did not provide more scientific explanations in stem-cell stories than web sites for broadcast and cable networks.