This report describes a method to estimate the number of migrant and seasonal farmworkers present in a prescribed area during crop harvest, and to pinpoint areas of high need for health and social services. The collection of health clinic and federal program data on migrant and seasonal farmworkers in Florida, northwestern Ohio, and Maryland's eastern shore demonstrated that the only available information of known quality and precision on which to base estimates is the Census of Agriculture, supplemented by special survey data. Accordingly, the suggested model utilizes crop acreage data, harvest season length, person hours needed to harvest a particular crop, and person hours worked per day, plus demographic information based on special surveys, migrant clinic data, Employment and Training Administration data, and Migrant Student Record Transfer System data. This model is simple and precise, makes use of data already available, can produce small area estimates, and is not affected by workers' immigration status. The model does not primarily produce demographic information, and errors may stem from outdated crop data or environmentally caused fluctuations in crop production. The report includes 10 references, 16 graphs and tables, appendices outlining the procedure for using this method, and estimates of migrant and seasonal farmworkers needed at harvest in 42 states, by country. (SV)