The objective of this work was to assess the effect of interferon beta-1a (Avonex) on the rate of development of clinically definite multiple sclerosis and brain magnetic resonance imaging changes in subgroups based on type of presenting event, baseline brain magnetic resonance imaging parameters, and demographic factors in the Controlled High-Risk Subjects Avonex Multiple Sclerosis Prevention Study (CHAMPS) trial. After the onset of a first demyelinating event, 383 patients with brain magnetic resonance imaging evidence of subclinical demyelination were treated with corticosteroids and randomly assigned to receive weekly intramuscular injections of 30 microg interferon beta-1a or placebo. The treatment effect within subgroups was assessed in proportional hazards models both for the development of clinically definite multiple sclerosis and for a combined outcome of development of clinically definite multiple sclerosis or >1 new or enlarging T2 lesions on brain magnetic resonance imaging. A beneficial effect of treatment was noted in all subgroups evaluated. Adjusted rate ratios for the development of clinically definite multiple sclerosis in the optic neuritis, brainstem-cerebellar, and spinal cord syndrome subgroups were 0.58 (p = 0.05), 0.40 (p = 0.03), and 0.30 (p = 0.01) and for the development of the combined clinically definite multiple sclerosis/magnetic resonance imaging outcome were 0.50 (p < 0.001), 0.41 (p = 0.001), and 0.40 (p = 0.004), respectively. A treatment benefit on both outcome measures also was seen in subgroups based on baseline brain magnetic resonance imaging parameters, gender, and age. Interferon beta-1a is beneficial when initiated at the first clinical demyelinating event in patients with brain magnetic resonance imaging evidence of subclinical demyelination. The beneficial effect is present for optic neuritis, brainstem-cerebellar syndromes, and spinal cord syndromes.