The safety and efficacy of pre- and coseasonal sublingual allergen immunotherapy (SLIT) with a 5-grass pollen sublingual tablet have been demonstrated in a randomized clinical trial (RCT) in children and adolescents. Observational, 'real-life' studies can usefully complement the results of RCTs.A prospective, open-label, observational, multicentre post-marketing study of children and adolescents (aged 5-17, with grass pollen-induced allergic rhinitis) treated with the 5-grass pollen sublingual tablet was performed between June 2009 and January 2011 in Germany. Adverse events (AEs) were recorded during consultations with the investigating physicians; AEs judged to have at least a possible causal link to the tablet were classified as adverse drug reactions (ADRs).Eight hundred and forty-nine patients were enrolled (by 207 investigating physicians), 829 (mean ± s.d. age: 10.9 ± 3.3 yr) completed the study without major protocol deviations, and 796 were fully documented with respect to AEs. Ninety-four of the 796 patients (11.8%) experienced at least one ADR on the first day of SLIT and 218 (27.4%) experienced at least one ADR during the study. Four hundred and sixty-six of the 596 ADRs (78.2%) were mild or moderate. The most common ADRs were throat irritation (19.1% of the reactions), oral paresthesia (8.2%), oral pruritus (6.5%) and oedema mouth (6.2%). Serious ADRs occurred in five patients. No epinephrine use was reported. Seventy-six of the 829 patients (9.2%) discontinued SLIT due to AEs. Tolerability was judged to be good or very good by patients (84.7%), parents (87.0%) and investigators (89.7%).In clinical practice, pre- and coseasonal treatment with a 5-grass pollen sublingual tablet is safe and well tolerated in children and adolescents.