To determine the value of algorithms in the field, a set of cardiac clinical algorithms was developed for and tested by the City of Philadelphia paramedics. A controlled test was carried out by revising the narrative standard operating procedure to reflect algorithm content and giving the algorithms, in pocketsize booklet form, to half the Fire Rescue platoons (43 men). Baseline data included paramedics' characteristics, arrhythmia recognition and management test scores, and data on the number of patients who were discharged alive from the hospital following successful treatment of ventricuiar fibrillation in the field. Platoons with and without the algorithms served identical areas of the city, did not differ in response times, and were treated similarly with regard to continuing education during the study period. Paramedics who received the algorithms were encouraged, but not required, to use them. Platoons using the algorithms experienced an increase in their ventricular fibrillation patient survival rates from 10.6% to 14.5%; those platoons not using the algorithms had a decrease in patient survival rates from 11.8% to 7.0% (P = 0.0732). There was no statistically significant difference in the paramedics' scores on arrhythmia recognition and management tests given before and after use of the algorithms. Only 39% of the paramedics used the algorithms during actual patient care, but 83% used them as a study guide or for reference between runs. The data suggest that algorithm use may minimize the rate of knowledge and skill decay.