Approximately 10% of patients with multiple sclerosis (MS) run a primary progressive course characterised by an accumulation of neurological deficits without relapse or remission. Designing therapeutic trials in primary progressive MS (PPMS) has presented several problems. Patient recruitment may be difficult because of the relative rarity of PPMS and historically has been hindered by the lack of specific diagnostic criteria. There has been a limited choice of validated outcome measures, although, in recent studies, the MS functional composite measure and magnetic resonance imaging measures of lesion load and atrophy have been widely used.Despite these problems, several trials have been designed specifically for PPMS, including exploratory randomised controlled trials of interferon-β-1a and interferon-β-1b and mitoxantrone, a phase III trial of glatiramer acetate, and an open-label study of riluzole. Patients with PPMS have also been included in randomised controlled trials of azathioprine, methotrexate, cladribine, intravenous immunoglobulin and cyclophosphamide, and open-label studies of haematopoietic stem cell transplantation and pirfenidone in progressive MS. However, no treatment has been proven definitively to modify the course of the disease. Looking to the future, therapeutic agents should aim to target the underlying pathogenic mechanisms in PPMS. As a result of the relative lack of inflammation in PPMS, neuroprotective agents that target neuronal loss directly, rather than inflammation, may be more worthwhile. However, further investigation into the pathogenic mechanisms in PPMS is required to guide the development of future therapeutic agents.