Auto-antibodies of the neuronal anti-nuclear antibody (anti-Hu) type were found in serum of three patients suspected of a paraneoplastic syndrome of the central nervous system. In all three a small cell carcinoma of the lung was detected. The sera showed bright staining of neuronal nuclei sparing the nucleolus. In two patients the antibody was of the IgG class (titers 1:1600 and 1:4000). In one patient only an IgM class antibody was present (titer 1:1000). The presence of the anti-Hu antibody strongly supports a diagnosis of a paraneoplastic neurological syndrome associated with small cell carcinoma of the lung.