Currently the best studied mechanism for amyotrophic lateral sclerosis (ALS) is the one caused by mutations in the gene for cytosolic Cu/Zn-binding superoxide dismutase (SOD1). Mutant SOD1 protein causes motor neuron degeneration due to the gain of a novel toxic function. To evaluate the relevance of SOD1 levels in cerebrospinal fluid (CSF) in ALS patients, the SOD1 concentration was immunoassayed in the CSF of 11 patients with ALS and 19 neurological controls. The mean level of SOD1 in CSF from all samples was 45.5+/-11.3 ng/ml. There was no statistically significant difference between the levels of SOD1 in CSF of ALS patients and neurological control subjects. Here we show that the SOD1 concentration in the CSF is significantly higher in male ALS patients (54.0+/-9.0 ng/ml) compared to female ALS patients (38.1+/-6.4 ng/ml) (p=0.007). This gender difference is not observed in the CSF of neurological controls. This is the first report of a potential gender difference in levels of SOD1 in CSF of ALS patients. Further investigation of larger sample groups is needed to determine whether it is relevant to gender related differences in disease incidence.