The aim of this study was to assess the influence of some risk factors for onset and remission of allergic rhinitis and asthma in Swedish adults. A random sample of 1,370 subjects, age 20 to 44 yr was investigated by means of postal questionnaires in 1990 and 1993. Skin prick tests were conducted in 1991-1992. The association between risk factors and onset or remission of allergic rhinitis and asthma was estimated using multivariate logistic regression analysis. Onset of allergic rhinitis was associated with sensitization to birch (odds ratio [OR] = 6.5), Parietaria (OR = 7.4); and pets (OR = 3.0) and with female sex (OR = 1.9). Onset of asthma was associated with allergic rhinitis (OR = 4.9), sensitization to pets (OR = 2.4); and with smoking (OR = 3.0). Onset of asthma was strongly associated with allergic rhinitis among atopics (OR = 5.7), but onset of asthma and rhinitis also tended to be related among nonatopics (OR = 3.5). A strong association between smoking and onset of asthma was found among nonatopics (OR = 5.7). In conclusion, sensitization to pollens and pets were risk factors for onset of allergic rhinitis, whereas allergic rhinitis, sensitization to pets, and smoking were risk factors for onset of asthma.