PURPOSE Anti-Hu antibodies (HuAb) recognize antigens expressed by neurons and small-cell lung cancer (SCLC). High titers of HuAb were initially reported in serum from patients with paraneoplastic encephalomyelitis/sensory neuropathy (PEM/SN) and SCLC. Preliminary studies have indicated that some SCLC patients without PEM/SN harbor low titer of HuAb in their serum, and that the SCLC of these patients may grow more indolently. Based on these observations, we conducted a multicenter prospective study of SCLC patients without PEM/SN to determine the incidence and prognostic implications of HuAb. METHODS Serum samples were collected at diagnosis of SCLC in 196 patients without PEM/SN. HuAb were determined by immunoblot of purified recombinant HuD antigen. RESULTS HuAb were detected in 32 (16%) of the 196 patients. Of the 170 patients who received treatment for the tumor, 27 (16%) were HuAb positive. HuAb was associated with limited disease stage (59.3% v 38.6%; P = .047), complete response to therapy (55.6% v 19.6%; P < .001), and longer survival (14.9 v 10.2 months; P = .018). In a logistic regression analysis, HuAb status was an independent predictor of complete response induction. The probability of achieving a complete response was more than five times higher in HuAb-positive than in HuAb-negative patients (odds ratio, 5.4; 95% confidence interval, 1.71 to 16.89; P = .004). Cox multivariate analysis indicated that HuAb status was not independently associated with survival. CONCLUSION The presence of HuAb at diagnosis of SCLC is a strong and independent predictor of complete response to treatment. This feature accounts for the association between HuAb and longer survival.