Assessment of efficacy in immunotherapy trials is difficult: a number of parameters can be measured; these may give conflicting results and it is not clear which best represents the clinical state. We therefore derived a Clinical Index (CI), by combining five individual parameters‐diary card symptom score, visual analogue symptom score, drug score, nasal challenge and skin prick test‐by rank analysis. The CI has been used to express the results in a trial of house dust mite (D. pteronyssinus) immunotherapy for allergic rhinitis, comparing two active treatments, Pharmalgen® and Allpyral®, with placebo. The derived index correlated well with all parameters measured. Correlations between the five individual parameters were variable: with the exception of the skin test, good correlations were seen, each parameter correlating with two or three of the others, but the visual analogue showed the closest association with other parameters. Results from a single parameter taken alone could be misleading, e.g. in the Allpyral group, nasal challenge improved significantly, but symptoms assessed either by visual analogue or diary card score did not. The CI showed significant improvement in the Pharmalgen group but not in the Allpyral group. The CI gave a more reliable measure of outcome than individual param