Table 1. NPC cases detected.

	VCA IgA status at enrollmenta
	Seropositive	Seronegative	Total
Number of subjects enrolled	3093	38 955	42 048
NPC cases detected at enrollment	39	1	40
Follow-up examinationsb	13186	9093	22 279
Number of NPC cases detected
By routine screeningc	34	0	34
In study clinic after onset of symptomsd	22	51	73
In other hospital after onset of symptomse	0	24	24
NPC case detected after enrollment	56	75	131
Incidence (cases/105 person-year)f	120.70	12.84	20.77
Relative riskg	5.81	0.62	1.00

NPC=nasopharyngeal carcinoma; VCA.IgA=viral capsid antigen IgA.

Study subjects were serologically screened and given a clinical examination at the time of enrollment.

^aSerum EBV VCA IgA titre ⩾1:5 designated seropositive, <1:5 designated seronegative.

^bPerformed on eight occasions over the subsequent 13 years either.

^cBy clinical examination conducted on these occasions.

^dwhen symptomatically presented to the clinic of our hospital.

^eTraced to other centers up to 15 years after enrollment.

^fCalculated over 15 years.

^gCalculated as ratios of cumulative incidence of seropositive or seronegative groups to both groups combined.

All NPC cases were WHO class II or III non-keratinizing carcinoma, except for five WHO class I cases of keratinized carcinoma.