Evidence on the use of clinical practice guidelines to aid in the legal determination of negligence is increasing, specifically where they affect determinations of the standard of care and causation. So too is evidence that some clinical practice guidelines are of poor quality. An action alleging the negligent failure to diagnose and treat gestational diabetes in 1988, in which the neonate suffered permanent brachial plexus injury, entered into evidence a 1984 clinical practice guideline authored by the Society of Obstetricians and Gynaecologists of Canada. No "experts" were called to adjudicate the quality of this guideline, which cited no evidence or rationale in support of its recommendations. The standard as laid out in the guideline was judged by the court to reflect a prevailing standard of care, and a finding of negligence was rendered. As the courts pay increased attention to clinical practice guidelines, critical appraisal by the professional organizations developing these documents must occur to assure methodological rigour. Further, the quality of clinical practice guidelines should receive critical scrutiny by the courts if they are to be relied upon, even partially, to assist with legal determinations of the standard of care or issues under causation.