Most of the controlled trials of specific immunotherapy (SIT) with house dust mite showed a good clinical efficacy of different kinds of extract after variable periods of treatment. However, few of them looked at all the parameters available to monitor this therapy, such as threshold dose of specific provocation test, end point titration and immunological variations. In particular, the behaviour of specific IgE antibodies is not completely known, while, in more recent studies, mite specific IgG antibodies have been shown to increase even in the early stages of treatment. Few data are available on mite specific IgG1-4 subclasses. The preliminary results of a double blind placebo controlled trial we performed with alginate conjugated Dermatophagoides pteronyssinus extract seem to indicate a good clinical and immunological response to this kind of extract. Moreover, we observed a significant increase of specific IgG antibodies, total and subclasses IgG1.2.4, with a prevalence of IgG4 subclass after three months of SIT. After two years of SIT mite specific IgG1 antibodies showed a tendency to decrease, while IgG4 antibodies maintained a constantly high level.