The Fcgamma receptors, FcgammaRIIA and FcgammaRIIIB contain polymorphisms with different capacity for IgG binding and phagocytosis. Thirty myasthenia gravis (MG) patients and 49 healthy controls were genotyped for the FcgammaRIIA and FcgammaRIIIB polymorphisms using polymerase chain reaction. The frequency of the FcgammaRIIA-H/H genotype was increased in thymoma MG patients compared to other MG patients (P = 0.05) and controls (P = 0.02). The distribution of FcgammaRIIIB alleles in MG patients did not differ from the controls, but MG patients with the NA1/NA1 genotype had the most severe MG (P = 0.01). Levels of AChR-antibodies and frequency of titin or ryanodine receptor antibodies were not associated with the FcgammaRIIA or FcgammaRIIIB genotypes. The results suggest different pathogenetic mechanisms in paraneoplastic and non-paraneoplastic autoimmune MG.