The Institute of Medicine concluded in To Err Is Human in 1999 that transformation of nurse work environments was needed to reduce patient harm. We studied 535 hospitals in four large states at two points in time between 2005 and 2016 to determine the extent to which their work environments improved, and whether positive changes were associated with greater progress in patient safety. Survey data from thousands of nurses and patients showed that patient safety remains a serious concern. Only 21 percent of study hospitals showed sizable improvements (of more than 10 percent) in work environment scores, while 7 percent had worse scores. For hospitals in which clinical care environments improved, patients and nurses reported improvements in patient safety indicators. These included increases in percentages of patients rating their hospital favorably (a change of 11 percent) and stating that they would definitely recommend the hospital (8 percent) and in percentages of nurses reporting excellent quality of care (15 percent) and giving the hospital a favorable grade on patient safety (15 percent). Where work environments deteriorated, fewer nurses (-19 percent) gave a favorable grade on patient safety. Failure to improve hospital work environments may be hampering progress on patient safety.