To determine the rate of appearance of new nodules during a specified period, a follow-up study on onchocercomas that had been removed was carried out in San Vicente Pacaya, Guatemala, between June 1976 and April 1977. Incidence rate, or rate of nodules appearing in initially negative subjects, was studied in relation to the degree of endemicity. Rates in a period of 7–8 months in high, medium, and low endemic areas were 0.231, 0.083, and 0.022, respectively. The rate of nodule appearance in persons nodulectomized, or rate of nodules appearing in subjects who were surgically rendered negative during the first survey, was 45.3%, 36.4%, and 4.5% in those areas. The highest rate of nodule appearance was observed in patients with microfilariae in both the skin and nodules, followed by patients with either microfilariae or nodules. In highly endemic areas, new nodules appeared in 13.2% of persons without microfilariae or nodules, while in low endemic areas even patients with nodules earlier were less likely to have developed new ones. The performance of the nodulectomy teams (brigadas) was evaluated by confirming the worms in nodules. By gross examination alone, about 6% of nodules removed by brigadas did not contain worms. Nodules containing worms were examined for microfilariae to clarify their role as a source of microfilariae. Microfilariae were not detected in some of these nodules, especially in small ones. In contrast, microfilariae emerged from 80% of those larger than 10 mm. Number of worms per nodule, their sex, and the fecundity of female worms were examined by a collagenase technique. The average number of worms per nodule was 0.6 males and 1.2 females. The highest burden in any one nodule was seven worms, four females and three males. Most nodules had one female and either one male worm or none. Intrauterine microfilariae were found in most females from nodules shared with a male, while solitary females were not gravid. Solitary females were likely to be found in small nodules and large nodules were more likely to include both sexes.