The mechanisms underlying the association between viral upper respiratory tract infections (URTIs) and asthma exacerbations are still unclear. This study was designed to investigate the effects of URTI on peripheral blood inflammatory cells and T-cells in atopic asthmatic patients. Initially, mild to moderate clinically stable atopic asthmatics and control nonatopic subjects were studied for baseline T-cell proliferation and cytokine secretion in response to allergen (house dust mite) and mitogen. This methodology was then used to study the effects of naturally acquired URTIs on the same parameters at the time of acute URTI (leading to asthma exacerbation in asthmatics) and during convalescence (14 days after). Respiratory viruses were detected in nasal secretions at the same time points. All the subjects were also restudied 2 months after acute URTI at a time when the asthmatics were stable. A further group of asthmatics with disease exacerbations requiring admission to hospital were also studied for the effect of asthma exacerbation on peripheral blood leucocytes, especially T-cells and their activation markers and adhesion molecules. These patients were classified as suffering from mild to moderate asthma and severe asthma. They were studied on admission, and after 7 and 56 days. I have found that PBMCs from atopic asthmatics have higher allergen specific proliferative responses than those from non-atopic control subjects with higher allergen-induced production of IL-5. I have found that HRV was the most common respiratory virus (32%) among the mile atopic asthmatic subjects with URTIs. In conclusion, I have found that URTIs have a profound effect on responses of peripheral blood T cells in asthmatic patients which may differ depending on the severity of the asthma exacerbation.