A multicenter, double-blind, placebo-controlled study was conducted to investigate the efficacy of an immunostimulant, Ribomunyl, in the prevention of recurrences of infectious rhinitis in adults. This trial involved 327 patients (168 Ribomunyl treated and 159 placebo cases) with an average of 4.3 +/- 1.8 rhinitis episodes per patient recorded during the year preceding the study. The main criterion of efficacy was the cumulative number of recurrences of infectious rhinitis during a 6-month follow-up period, as analyzed by standard tests. An additional analysis of relative risk of recurrences used multivariate failure for time data. Ribomunyl was effective throughout the study period, starting from the first month of treatment: a mean of 1.0 +/- 1.1 recurrences was recorded in the Ribomunyl group as compared to 1.5 +/- 1.4 recurrences in the placebo group; this indicated one-third fewer infections (P = 0.001). The protective effect of Ribomunyl on the relative risk for recurrences was estimated to be 0.58 by multivariate analysis (95% CI: 0.43-0.78, P = 0.0001). Analysis of secondary criteria also favored Ribomunyl: 38.5% less antibiotic courses per patient (0.8 +/- 1.3 vs 1.3 +/- 1.6; P = 0.002) and the number of days with antibiotics (5.6 +/- 9.3 vs 9.1 +/- 12.1; P = 0.002).