The objective of this study was to evaluate the effect of intra-articular corticosteroid injection (IACI) of methylprednisolone acetate (MPA) on the hypothalamic-pituitary-adrenal (HPA) axis in patients with osteoarthritis of the knee. Patients with symptomatic osteoarthritis of the knee who failed to respond to nonsteroidal anti-inflammatory medications and physical therapy were randomized between group 1 and group 2. Group 1 patients had an IACI of 80 mg of MPA at the knee joint and group 2 patients had an intra-articular injection (IAI) of 6 ml (60 mg) of sodium hyaluronate (control group). Immediately prior to the IAI and on weeks 1, 2, 3, 4, and 8 following IAI, patients from both groups underwent a low-dose (1 μg) adrenocorticotropin hormone (ACTH) stimulation test. Demographic, clinical, laboratory, and radiologic variables were documented in all patients. Both criteria of <7 μg/dl increase in the serum cortisol level and absolute levels of <18 μg/dl 30 min following the ACTH stimulation test were used to define secondary adrenal insufficiency (SAI). Twenty patients were randomized in each group. In group 1, 25 % of patients had SAI vs. none in group 2 (p = 0.0471). The earliest SAI was observed at week 2, and latest SAI was observed at week 4. SAI was observed at one time point, two consecutive time points, or two separate time points in the same patient. There was no correlation between SAI and any of the demographic, clinical, or laboratory variables. An IACI of 80 mg MPA at the knee joint induced a transient SAI in 25 % of the patients, an effect that was observed between week 2 and week 4 following the IACI.