Phagocytosis of bacteria constitutes an important defense mechanism against invasive bacterial diseases. Efficacy of phagocytosis by polymorphonuclear neutrophils is known to vary between allotypes of Fc gamma RIIa (a class of Fc receptors for immunoglobulins that is constitutively expressed on neutrophils). We compared the distribution of Fc gamma RIIa-R131 and Fc gamma RIIa-H131 allotypes in 98 Slavic complement-sufficient patients with meningococcal disease with that of the allotypes in 107 healthy controls. A strong association was found between the IIa-R/R131 allotype and the development of meningococcal disease after the age of 5 years, compared with IIa-R/H131 and IIa-H/H131 allotypes (P < .03; odds ratio [OR], 2.9). A severe course of meningococcal disease was observed in 21 (68%) of 31 episodes in patients with IIa-R/R131 genotype and in 22 (54%) of 41 episodes in patients with IIa-R/H131 genotype, in contrast to eight (31%) of 26 episodes in patients with IIa-H/H131 genotype (P < .02; OR, 4.7). Our data show that individuals older than 5 years of age who have the IIa-H/H131 allotype are less susceptible to severe meningococcal disease than are individuals with the IIa-R/R131 or IIa-R/H131 genotype.