We sought to characterise adolescent wheeze in the absence of asthma, which we termed "undiagnosed wheeze". The Isle of Wight Birth Cohort (n=1,456) was reviewed at 1, 2, 4, 10 and 18 yrs. Using questionnaire responses, "asthma" was defined as "ever had asthma" plus either "wheezing in the last 12 months" or "taking asthma treatment in the last 12 months"; "undiagnosed wheeze" as "wheeze in the last 12 months" but "no" to "ever had asthma"; and remaining subjects termed "non-wheezers". Undiagnosed wheeze (prevalence 4.9%) accounted for 22% of wheezing at 18 yrs. This was largely adolescent-onset with similar symptom frequency and severity to diagnosed asthma. However, undiagnosed wheezers had significantly higher forced expiratory volume in 1 s to forced vital capacity ratio, less bronchodilator reversibility and bronchial hyperresponsiveness, and were less frequently atopic than asthmatics. Undiagnosed wheezers had earlier smoking onset, higher smoking rates and monthly paracetamol use than non-wheezers. Logistic regression identified paracetamol use (OR 1.11, 95% CI 1.01-1.23; p=0.03), smoking at 18 yrs (OR 2.54, 95% CI 1.19-5.41; p=0.02), rhinitis at 18 yrs (OR 2.82, 95% CI 1.38-5.73; p=0.004) and asthmatic family history (OR 2.26, 95% CI 1.10-4.63; p=0.03) as significant independent risk factors for undiagnosed wheeze. Undiagnosed wheeze is relatively common during adolescence, differs from diagnosed asthma and has strong associations with smoking and paracetamol use. Better recognition of undiagnosed wheeze and assessment of potential relevance to adult health is warranted.