Posterolateral rotatory instability (PLRI) of the elbow occurs from attrition of the lateral ulnohumeral collateral ligament of the elbow after elbow dislocation. Diagnosis by physical examination can be difficult in the awake patient. The goals of this study were to define two active apprehension signs for the physical diagnosis of PLRI and to perform a prospective evaluation of the signs in a series of patients with PLRI. Eight patients with PLRI undergoing surgical reconstruction of the lateral ulnocollateral ligament of the elbow were prospectively included in this continuous case series. Preoperative evaluation consisted of physical examination with two active apprehension signs, the chair sign and the pushup sign, as well as the pivot-shift sign. Results were compared with repeat physical examination after reconstruction of the ligaments. Of 8 patients included in the series, 3 demonstrated a positive pivot-shift sign while awake, and all demonstrated a positive pivot-shift sign while under anesthesia. Seven patients demonstrated a positive chair sign, and seven demonstrated a positive pushup sign. At the 2-year follow-up evaluation, 7 patients remained stable and asymptomatic. The pushup sign, chair sign, and pivot-shift sign were negative in all 7 patients. The study demonstrated that both the pushup and chair signs are effective in aiding the diagnosis of PLRI. They are more sensitive than the pivot-shift sign in the awake patient and may be easily performed in the office environment.