The objective of the study was to evaluate SLEDAI-2K 30 days over time and to compare with the original SLEDAI-2K 10 days. Forty-one patients seen at The University of Toronto Lupus Clinic were followed at monthly intervals for 12 months. The SLEDAI-2K score was completed twice, once for a 10-day window and again for a 30-day window using the same definitions for the descriptors. Four hundred and nineteen patient-visits in 41 patients were recorded for both SLEDAI-2K for a 10-day and a 30-day window. One hundred and fifty-one patient-visits had a SLEDAI-2K activity score of 0 and 268 patient-visits had varying levels of disease activity in the range 1-15. In all but one patient-visit there was an agreement between the SLEDAI-2K 10 days and 30 days. SLEDAI-2K 30 days scores were concordant with SLEDAI-2K 10 days scores, both in patients in remission and in patients with a spectrum of disease activity levels followed monthly over 1 year. SLEDAI-2K 30 days was validated against SLEDAI-2K 10 days in a longitudinal evaluation over 1 year. We recommend the use of SLEDAI-2K 30 days in clinical studies and clinical trials.