On investigation of 101 attacks in 24 patients with hereditary neuralgic amyotrophy (HNA) from nine different families, we found that HNA can run two distinct courses: a 'classic' relapsing-remitting and a chronic undulating type with exacerbations. Only one type occurred per family, suggesting genetic heterogeneity. This is supported by the finding that only in a family with 'classic type' HNA are data of linkage analysis compatible with linkage to the 17q24-q25 interval which harbours a locus for the disease. The average number of attacks per patient during a follow-up of 26 years was four in the classic form of HNA and five in the chronic undulating type. All patients suffered from residual symptoms on follow-up, with a median Rankin score of 2 in both groups, showing that long-term prognosis is less favourable than previously reported.