Influenza is an acute febrile illness caused by influenza A and B virus affecting all age groups. Influenza occurs annually and each year it is estimated that influenza epidemics cause 36,000 deaths and 114,000 hospitalizations in the US alone. Influenza infections are generally self-limiting; however significant morbidity and mortality can occur, predominantly in high risk groups such as the elderly and those suffering from chronic conditions. This study is a Phase I, double-blind, placebo-controlled, ascending single inhaled dose, safety, tolerability and pharmacokinetic study in elderly subjects with stable health status. The primary objective of this study is to evaluate the safety and tolerability of CS-8958 (5, 10 20 and 40 mg) in elderly subjects after a single dose administered via inhalation. Primary outcome measures of safety and tolerability will be addressed in terms of occurrences of treatment-emergent adverse events (AEs), changes in vital signs including blood pressure and pulse rate, functional oxygen saturation of arterial hemoglobin, oral body temperature, electrocardiogram, spirometry \[Forced Vital Capacity (FVC)\], Forced Expiratory Volume in 1 second (FEV1), Forced Expiry Volume percentage in 1 second (FEV1.0%), Forced Expiratory Flow Rate (FEF) 25%-75%), Peak Expiratory Flow Rate (PEFR, also called PEF), physical examinations, psychological assessments, and laboratory parameters. The secondary objectives of this study are to assess the systemic exposure of CS-8958 and its active metabolite R-125489 after single inhaled doses of CS-8958 in elderly subjects. The secondary outcome measures of pharmacokinetic parameters will be calculated for CS-8958 and R-125489 (active metabolite) concentrations in plasma and urine using a non-compartmental approach. These measurements will be used to assess the systemic exposure of CS-8958 and its active metabolite in elderly subjects after a single inhaled dose of CS-8958 (5, 10, 20 and 40 mg). This data will help to determine safety margins for dosing of elderly subjects in future studies. Thirty-eight healthy adults (male and female), aged 65 years and older, with stable health status, will be recruited for this UK based study. Subjects will be studied in 4 groups (Groups A to D) with each group consisting of at least 8 subjects. Within each group, subjects will be randomly allocated to Arm 1 or Arm 2. Arm 1 will receive the investigational drug with lactose to 25 mg (placebo) and Arm 2 will receive placebo only. Treatment will begin with a low dose of 5 mg (Group A) and subsequent higher doses of 10, 20 and 40 mg (Group B, C and D, respectively) It is planned that doses will be administered in an escalating manner. The Safety Monitoring Committee will review the safety, tolerability and pharmacokinetic data from each group prior to administration of the next dose. Following these reviews, the dose increment for subsequent groups may be increased, or the doses administered may be reduced, and may be lower than the starting dose. The maximum dose level to be studied will be 40 mg. The duration of the study for each subject will be a maximum of 6 weeks from initial screen to the follow-up visit. The screening period will be at most 4 weeks followed by 7 days in the clinic and a follow up visit 5-7 days after leaving the clinic.