The purpose of this study is to determine the safety and effectiveness of an investigational drug for the treatment of cognitive impairment in patients with schizophrenia.

• Paranoid Schizophrenia
• Schizophrenia

• Drug: MK0557
• Drug: Comparator: Placebo (unspecified)

• Other: MK0557 10mg tablet qd, crossing over to MK0557 Pbo tablet qd.
• Other: MK0557 Pbo tablet qd, crossing over to MK0557 10mg tablet qd.

Inclusion Criteria:

• Patient is clinically stable, on current antipsychotic medication for at least 3 months and current dose for 2 months
• Patient has a 6th grade reading level or better and has completed at least 6 years of formal education
• Females are not pregnant, and those who can have children agree to remain abstinent or use acceptable birth control throughout study
• Patient has had a stable living arrangement for at least 3 months prior to study start
• Patient is in general good health based on screening assessments

Exclusion Criteria:

• Patient has a major disease/disorder that may interfere with cognitive testing (such as mental retardation) and/or pose a risk upon study participation
• Patient has a history of head trauma with loss of consciousness greater than 15 minutes
• Patient has had warfarin treatment, MAO inhibitors, clonazepam, clozapine or St. John's wort within 1 month of screening
• Patient has had ECT treatment within 6 months of screening
• Patient requires treatment with antihistamines or certain other medications listed in the protocol
• Patient has a history of liver disease that has been active within the last 2 years, or a history of cancer within past 5 years
• Patient has a history of alcohol or drug dependence within past year or alcohol or drug abuse within 3 months of screening