Acute partial transverse myelitis (APTM) may be the first clinical manifestation of multiple sclerosis (MS), of relapsing myelitis, or remain a monophasic event. Identification of risk factors associated with relapse or conversion to MS is important, as prognostic information might help to guide management. The objective of this study was to define clinical, laboratory and neuroimaging factors in patients with first-ever APTM that predict relapses or conversion to MS. We identified 73 patients with a first-ever APTM admitted to our institution from January 1999 to June 2005. The follow-up time ranged from 12 to 90 months (mean follow-up 46 months). Patient demographics, clinical impairment at onset and after 3 months, ancillary tests including cerebrospinal fluid (CSF), magnetic resonance imaging (MRI), evoked potentials, recurrent and new symptoms and signs during follow-up were analysed. APTM remained a monophasic event in 35 patients (47.9%), conversion to MS occurred in 32 (43.8%) and recurred as relapsing myelitis in six patients (8.2%). According to univariate analysis, a family history of MS (P = 0.02), higher expanded disability status scale (EDSS) at onset (P = 0.03) and lesions on brain MRI (P = 0.03) were predictive factors for conversion to MS. CSF-specific oligoclonal bands (P = 0.04) or abnormal IgG-index (P = 0.04) were associated with increased risk for MS as well. In patients with a first-ever APTM, a family history of MS, high EDSS at presentation, lesions on brain MRI, CSF-specific oligoclonal bands or abnormal IgG-index may indicate an increased risk for conversion to MS.