Following a systematic review of literature describing the relationship between the use of clinical practice guidelines (CPGs) and length of stay (LOS), we conducted a secondary analysis of the same relationship using data from Ontario acute care hospitals over two years. Unlike the original systematic review, this analysis did not find a strong relationship between CPG usage and LOS: the relationship between CPG usage and LOS was statistically significant only in the disease states of pneumonia and prostatectomy. Possible reasons for this outcome are explored, as are limitations, policy implications and suggestions for future research.