A morning lymphocytosis has been observed in patients under methylprednisolone (mPDN) treatment. We here determine prospectively the timing and magnitude of mPDN-induced lymphocytosis and study the effects of concomitant beta-blockers administration on lymphocyte count (L). L was measured before and 24 to 72 hours after initiating mPDN treatment in 20 patients with immune-mediated inflammatory disorders (IMID). After one week, patients with increased L were divided in two groups receiving, in addition to mPDN, either propranolol or a placebo; L was determined 4 days later. Lymphocyte subpopulations and mPDN plasma levels were determined in subsets of the patients. Values are expressed as median with 25%-75% interquartile range. A 73.4 % (37-305) increase of L was observed in 18/20 patients as soon as 48 (48-72) hours after initiating mPDN (32 mg; 16-32). Lymphocytosis (L ≥4,000/µL) was observed in 7 patients and hyperlymphocytosis (L ≥ 5,000/µL) in 4 of them. No significant changes in L variation were shown under propranolol. In conclusion, the morning lymphocytosis observed during mPDN treatment occurs in the very first days of mPDN administration. The lack of effect of propranolol does not support the role of an increased adrenergic tone.