Objective: to define statistically the upper limit of the inter-movement interval (IMI, the time interval between the onset of consecutive movements) of periodic leg movements during sleep (PLMS). Subjects and Methods: we computed the IMI distribution of a large sample (n = 141) of patients with restless legs syndrome (RLS) and analyzed it with two independent approaches, based on fitting either empirical functions or normal and exponential functions to the data. Results: the two fitting approaches consistently pointed to an upper limit of the PLMS IMI in the range between 50 s and 60 s. Decreasing the upper limit of PLMS IMI from 90 s to 60 s evidently decreased the PLMS index in RLS patients and control subjects; nevertheless, the PLMS index remained significantly higher in RLS vs. control subjects. Shifting the upper limit of PLMS IMI to 60 s did not significantly modify the effectiveness of discrimination of PLMS between controls and RLS patients. Conclusion: these results seem to indicate that a conservative, yet data-driven upper limit for IMI contributing to the PLMS in patients with RLS might be 60 s instead of 90 s, as recommended by the present guidelines.