Limited information exists regarding the incidence and predictors of asthma and nasal allergy in adulthood. We determined the incidence rate of asthma and nasal allergy in adults and assessed the predictive value of skin prick tests (SPTs) and radioallergosorbent tests (RASTs) for these two outcomes. Two German centres involved in the European Community Respiratory Health Survey conducted a follow-up assessment in 2012 of the baseline participants (1185 adults aged 21-47 years assessed in 1990). The predictive value of SPTs and RASTs on new-onset asthma and nasal allergy was assessed by Cox regression and by calculating the positive or negative predictive value. During the 20 years between baseline and follow-up, 3.1 and 4.4 per 1000 person-years of new-onset asthma and nasal allergy cases were recorded, respectively. The hazard ratios for SPTs of any specific and of all aeroallergens combined were slightly higher than those of RASTs for asthma and nasal allergy. The negative predictive values of both the SPT and RAST were very high and similar (0.94-0.96), whereas the postive predictive values were low (0.09-0.20). Positive SPT results showed a better association with new onset asthma and nasal allergy than positive RAST either to any specific aeroallergens or to all combined.