Familial amyotrophic lateral sclerosis (fALS) is a well-recognised condition that accounts for almost 10% of all cases of ALS. Most cases are now known to be transmitted by an autosomal dominant trait. When fALS is compared clinically to sporadic ALS, 20% of cases manifest atypical features such as pain, paraesthesia or urgency micturition. Moreover, a disease duration of over 10 years, with very slow progression, appears to occur almost exclusively in cases of fALS. Studies of superoxide dismutase (SOD1) mutations in fALS have shown that the disease may be multidegenerative, with oculomotor or cerebellar involvement. Molecular genetics has also demonstrated that not all SOD1 mutations have a dominant influence, and the detailed description of the Scandinavian D90A homozygous mutation is very informative in this regard. Misdiagnosis of fALS can be attributed to one of the following situations: (i) atypical phenotype ALS with a multidegenerative profile; (ii) unusually long lasting ALS with mild motor neuron involvement; (iii) significant clinical heterogeneity between affected family members; (iv) low reliability of family history; (v) existence of an unknown or unexpected mode of transmission; and (vi) other multidegenerative disorders with motor neuron involvement. Pedigrees and fALS cases corresponding to these situations are presented.