Minocycline is proposed as an add-on therapy to improve the efficacy of glatiramer acetate in relapsing-remitting multiple sclerosis. The effect of minocycline plus glatiramer acetate was evaluated in this double-blind, placebo-controlled study by determining the total number of T1 gadolinium-enhanced lesions at months 8 and 9 in patients who were starting glatiramer acetate and had at least one T1 gadolinium-enhanced lesion on screening magnetic resonance imaging. Forty-four participants were randomized to either minocycline 100 mg twice daily or matching placebo for 9 months as add-on therapy. They were assessed at screening and months 1, 3, 6, 8 and 9. Forty participants completed the study. Compared with glatiramer acetate/placebo, glatiramer acetate/minocycline reduced the total number of T1 gadolinium-enhanced lesions by 63% (mean 1.47 versus 2.95; p = 0.08), the total number of new and enlarging T2 lesions by 65% (mean 1.84 versus 5.14; p = 0.06), and the total T2 disease burden (p = 0.10). A higher number of gadolinium-enhanced lesions were present in the glatiramer acetate/minocycline group at baseline; this was incorporated into the analysis of the primary endpoint but makes interpretation of the data more challenging. The risk of relapse tended to be lower in the combination group (0.19 versus 0.41; p = NS). Treatment was safe and well tolerated. We conclude that efficacy endpoints showed a consistent trend favoring combination treatment. As minocycline is a relatively safe oral therapy, further study of this combination is warranted in relapsing-remitting multiple sclerosis.