To evaluate the effectiveness of the Volunteer Collaborator Network (VCN) of Latin America as a community-based malaria case detection and treatment system, we conducted a study of the VCN of Guatemala. Volunteer Collaborators took 72.6% of all blood smears and identified 81.3% of all malaria cases reported by the Guatemalan National Malaria Service. The average volunteer treated 5.8 patients per month (range 0-32.8). In contrast, passive case detection (PCD) posts in government hospitals and health centers treated an average of 12.5 patients per month (range 0.5-91.4). The slide positivity rate of blood smears taken by Volunteer Collaborators was 16.2% compared with 9.7% for PCD posts in health centers and 10.3% for malaria workers during active case detection. The average delay between the date a blood smear was taken and examined ranged from 18.1 days on the Pacific coastal plain to 26.3 days in the less accessible northern region of the country. An additional 14.5 to 47.6 days elapsed before the radical treatments were initiated in these two regions. Seventy percent of the patients completed their radical treatments. In a survey conducted on the Pacific coastal plain of Guatemala, of 1,021 patients with chills and/or fever who believed they had malaria, 20.0% had visited a Volunteer Collaborator and 4.9% were treated at a government health center. Thus, the PCD network detected only 25% of all patients with symptoms suggestive of malaria. Most of the remaining patients treated themselves with antimalarial medications purchased in stores and pharmacies, but less than 15% of these patients used adequate courses of therapy. Furthermore, the rate of detection of symptomatic patients with malaria varied considerably from one community to another. Thus, data from the VCN are probably most useful when groups of communities or geographic areas are stratified for malaria control activities because at this level, variations between individual Volunteer Collaborator posts will be minimized. In spite of these problems, the VCN remains an excellent source of epidemiologic data for malaria control programs and the most practical means available for providing timely, appropriate antimalarial therapy to febrile patients in rural areas.