OBJECTIVE: The objective of this study was to compare glucose control in participants with type 1 diabetes receiving insulin glargine 300 units/mL (Gla-300) or glargine 100 units/mL (Gla-100) in the morning or evening, in combination with mealtime insulin. RESEARCH DESIGN AND METHODS: In this 16-week, exploratory, open-label, parallel-group, two-period crossover study (clinicaltrials.gov identifier NCT01658579), 59 adults with type 1 diabetes were randomized (1:1:1:1) to once-daily Gla-300 or Gla-100 given in the morning or evening (with crossover in the injection schedule). The primary efficacy end point was the mean percentage of time in the target glucose range (80-140 mg/dL), as measured using continuous glucose monitoring (CGM), during the last 2 weeks of each 8-week period. Additional end points included other CGM glycemic control parameters, hypoglycemia (per self-monitored plasma glucose [SMPG]), and adverse events. RESULTS: The percentage of time within the target glucose range was comparable between the Gla-300 and Gla-100 groups. There was significantly less increase in CGM-based glucose during the last 4 h of the 24-h injection interval for Gla-300 compared with Gla-100 (least squares mean difference -14.7 mg/dL [95% CI -26.9 to -2.5]; P = 0.0192). Mean 24-h glucose curves for the Gla-300 group were smoother (lower glycemic excursions), irrespective of morning or evening injection. Four metrics of intrasubject interstitial glucose variability showed no difference between Gla-300 and Gla-100. Nocturnal confirmed (<54 mg/dL by SMPG) or severe hypoglycemia rate was lower for Gla-300 participants than for Gla-100 participants (4.0 vs. 9.0 events per participant-year; rate ratio 0.45 [95% CI 0.24-0.82]). CONCLUSIONS: Less increase in CGM-based glucose levels in the last 4 h of the 24-h injection interval, smoother average 24-h glucose profiles irrespective of injection time, and reduced nocturnal hypoglycemia were observed with Gla-300 versus Gla-100.