Entamoeba histolytica is the most common pathogenic intestinal parasite in México. Prevalence rates of AM vary widely from place to place, being higher in areas with poor sanitation, households and institutions, and among male homosexuals. Early recognition and treatment may be necessary to prevent its spread in the community. Intervention by preventive health education is indicated. The timing of such educational endeavours may be adjusted to any rhythmic and to that extent predictable changes in the incidence of AM and of any other possibly related gastrointestinal parasitic disease, such as GD. Accordingly, monthly totals of cases of AM and GD detected in Monterrey, (México) between November, 1985, and October, 1987, were fitted by linear least-squares with a 1-year cosine curve. The no-circannual rhythm assumption in the incidence of AM and GD was rejected in both cases (P = .006 and .049, respectively). A comparison of circannual parameters shows differences in M, A and phi between the incidences of AM and GD, the Ps for those comparisons being less than .001, .014 and .041, respectively. A high incidence of GD (above the yearly mean) consistently observed in April could be, however, related to the circannual pattern of AM, showing higher incidences in the spring. The timing of any preventive intervention should take into consideration the circannual variability in the incidence of AM and the differences or correlations with the circannual patterns of other gastrointestinal parasitic diseases.