With the purpose of evaluating the usefulness of clinical data and a coagglutination test for the diagnosis of streptococcal pharyngeal tonsillitis, a study was carried out comparing both methods with the standard pharynx culture (gold standard). One hundred and forty-one patients clinically diagnosed with pharyngeal tonsillitis were analyzed. 80 other patients diagnosed with rhino-pharyngitis were used as control groups as well as a group of 66 healthy patients. It was found that the presence of pus in the tonsils and the absence of rhinorrhea and/or a productive cough were isolated clinical variables which showed a significant difference when comparing those cases with a positive or a negative culture (P less than 0.01). These clinical data had a sensitivity and adequate negative predictive values (68 to 83% and 90 to 92%, respectively); specificity was 44 to 74% and the positive predictive value from 24 to 36%. The gathering of combined clinical indexes with different symptoms and signs, were not greater than isolated data. The coagglutination test in those cases of pharyngeal tonsillitis showed greater better specificity (98%) and positive predictive value (93%), but saw no improvement in sensitivity (72%) nor in the negative predictive value (91%), which is why when faced with a negative coagglutination test, it is still necessary to perform a throat culture due to the risk which the patient may be exposed to when no treatment is given. The analysis of the advantages and disadvantages in the routine use of coagglutination and cultures in their particular cases, allows us to conclude that in daily clinical practice the use of clinical criteria is of greater usefulness.(ABSTRACT TRUNCATED AT 250 WORDS)