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The ability of Plasmodium falciparum-infected red blood cells (RBC) to form spontaneous erythrocyte rosettes was studied in 130 fresh isolates from Gambian children with cerebral or uncomplicated malaria from August to November 1990. All isolates (24 of 24) from patients with cerebral malaria formed rosettes, but only 61 of 106 isolates from children with uncomplicated malaria formed rosettes. The mean rate of rosette formation in isolates from children with cerebral malaria (28.3%) was significantly greater than that in isolates from children with uncomplicated malaria (8.5%). Giant rosettes were more frequently formed in isolates from patients with cerebral malaria than in those from patients with uncomplicated malaria. Sera of children with cerebral disease generally lacked anti-rosette activity, while many sera from children with uncomplicated malaria showed strong anti-rosette activity when tested against the patients' own parasites. Some sera that were devoid of autologous rosette-disrupting activity were able to disrupt rosettes formed in other isolates, indicating the presence of different rosette formation mechanisms. Forty percent (6 of 15) of the sera from patients with cerebral malaria caused microagglutination of the patients' own uninfected and infected RBC, while only 10% (3 of 31) of sera from children with uncomplicated disease caused microagglutination. The ability of infected RBC to bind to melanoma cells grown in vitro did not differ between patients with cerebral or uncomplicated malaria. The results of this study, taken in conjunction with our previous findings, establish a strong association between rosette formation in P. falciparum-infected RBC and cerebral malaria.