Giardia lamblia, once considered a harmless commensal organism, has become one of the most common pathogenic intestinal parasites. Evidence for various methods of transmission has accumulated and serious physiological and nutritional disturbances as a result of infestation with this parasite have been clinically documented. Giardia lamblia has now been identified as a causative agent of waterborne, foodborne and sexually transmitted infectious diarrhea. Preventive interventions by health education (especially in personal hygiene) are indicated to reduce its spread in the community. The timing of such educational endeavours may be adjusted to the changes in the incidence of giardiasis if such should be predictable insofar as they are rhythmic. Accordingly, monthly totals of positive detected cases of giardiasis reported in Mexico between 1977 and 1985 were first fitted by linear least-squares with a 1-yr cosine curve. Results indicate a predictable circannual variability of Giardia incidence (P less than 0.001), with a crest time situated on the third week of July and monthly means of detected cases above the yearly average incidence between May and September. The validity of this circannual rhythm was further checked by nonlinear least-squares. Results show a presumably 1-yr synchronized estimated period of 8715.1 hr, with a total predictable change (double amplitude of 1788 cases per month) of 43% the average monthly incidence. Moreover, circannual rhythm parameters computed separately for each consecutive year are similar in terms of acrophase (P = 0.771). Awareness of the thus detected circannual predictable variability in the incidence of giardiasis may be important in the prevention, diagnosis and treatment of this infectious disorder. According to the results here found, the timing of prevention interventions by health education should be a readily exploitable factor.