Recently, the clinical efficacy of interferon beta-1b (IFNbeta-1b) was demonstrated for secondary progressive (SP) multiple sclerosis in a European multicentre study. We evaluated the effect of IFNbeta-1b treatment on the rate of development of hypointense T(1) MRI lesions, a putative marker of axonal damage. Unenhanced T(1)-weighted images were obtained in a subgroup of 95 multiple sclerosis patients from five centres at 6-month intervals; this subgroup was similar to the total study population for all demographic, clinical and MRI parameters. An experienced observer blinded to the clinical data and treatment allocation measured volumes. The median baseline lesion load for hypointense T(1) lesions was 5.1 cm(3) for placebo-treated and 4.9 cm(3) for IFNbeta-1b-treated patients (P = 0.56). Placebo-treated patients showed an increase in T(1) lesion load by a median of 14% per year (P = 0.0002 compared with baseline); this was reduced to 7.7% per year in the IFNbeta-1b-treated patients (P = 0.003 versus placebo). In the IFNbeta-1b arm there was a statistically significant correlation between absolute change in Expanded Disability Status Scale scores and T(1) lesion load by month 36 (r = 0.38, P = 0.0015). In patients with SP multiple sclerosis, IFNbeta-1b treatment reduces the development of hypointense T(1) lesions, suggesting that reduced axonal damage in lesions may play a part in the beneficial effect that is observed clinically.