Sublingual immunotherapy (SLIT) with monomeric carbamylated allergoid administered in accordance with the standard regimen has proven to be effective and safe. Achieving clinical benefit, however, requires a lengthy period of time so it is not very suitable for short-lasting allergies. We thus performed this study to compare an administration protocol starting in the coseasonal period (with a 4-day build-up phase) with a precoseasonal scheme to verify if the former regimen provides the same benefit in a shorter period of time. The prospective, randomized, drug therapy-controlled study was conducted in 33 rhinitic patients monosensitized to Olea with or without asthma. Ten patients were assigned to the coseasonal therapy with 5000 allergic units (AU)/week for 6 weeks, 11 to the precoseasonal therapy with 3000 AU/week for 10 weeks, and 12 to drug therapy. They were treated from April or May to June 2008. A visual analog scale (VAS) was performed at baseline and after treatment to assess the well being of the patients. Drug consumption was evaluated by means of a monthly diary. There was greater VAS improvement in both the SLIT groups versus the controls, but it was statistically significant only in the coseasonal group (p < 0.01). Furthermore, there was a reduction in the rescue medication only in the coseasonal SLIT (p < 0.05 versus drug therapy). One mild adverse event was observed. The allergoid SLIT was shown to be effective and safe in Olea allergy in particular when a coseasonal regimen was used.