The clinical course of infectious mononucleosis was monitored in 122 patients to determine predictors of duration of illness. Gastrointestinal symptoms and palatal petechiae were associated with a prolonged recovery period. Multiple regression analysis of quantitative data identified the number of days from onset of symptoms to diagnosis as the only statistically significant indicator. Liver enzymes, white blood cell count, percentage atypical lymphocytes and heterophil titer had no prognostic value for the length of illness.