Objective: To determine clinically related characteristics in patients with pure lower motor neuron (LMN) syndromes, not fulfilling accepted diagnostic criteria, who were likely to respond to intravenous immunoglobulin (IVIg) treatment. Methods: Demographic, clinical, laboratory and neurophysiological characteristics were prospectively collected from patients with undifferentiated isolated LMN syndromes who were then treated with IVIg. Patients were classified as either responders or non-responders to therapy with IVIg based on clinical data and the two groups were compared. Results: From a total cohort of 42 patients (30 males, 12 females, aged 18-83 years), 31 patients responded to IVIg and 11 did not. Compared to patients that developed progressive neurological decline, responders were typically younger (45.8 compared to 56.0 years, P<0.05) and had upper limb (83.9% compared to 63.6%, NS), unilateral (80.6% compared to 45.5%, P<0.05), and isolated distal (54.1% compared to 9.1%, P<0.05) weakness. Patients with predominantly upper limb, asymmetrical, and distal weakness were more likely to respond to IVIg therapy. Of the patients who responded to treatment, only 12.9% had detectable GM 1 antibodies and conduction block (not fulfilling diagnostic criteria) was only identified in 22.6%. Conclusions: More than 70% of patients with pure LMN syndromes from the present series responded to treatment with IVIg therapy, despite a low prevalence of detectable GM 1 antibodies and conduction block. Patients with isolated LMN presentations, not fulfilling accepted diagnostic criteria, may respond to IVIg therapy, irrespective of the presence of conduction block or GM 1 antibodies, and should be given an empirical trial of IVIg to determine treatment responsiveness.