This paper examines the structure, validity, and reliability of Guglielmino's self-directed learning readiness scale (SDLRS). It begins by reviewing claims made in the literature that the scale is sound, valid and reliable, and attempts to trace the source of these claims. This leads to scrutiny of Guglielmino's original thesis, which is found to be marred by both conceptual and methodological problems. The paper then reports data derived from an administration of the scale to students undertaking diplomas in vocational teaching or training. These data reveal a number of problems in the wording of the scale and show that the scale appears to measure a homogeneous construct that does not appear to be associated with readiness for self-directed learning. The paper concludes that the scale is structurally unsound and invalid.