A report on a series of patients having the outcome of interest. It is a descriptive observational study and does not usually have a control group.

Reports of case series usually contain detailed information about individual patients, including demographic information (age, gender, ethnic origin, etc.) and information on diagnosis, treatment, response to treatment, and follow-up after treatment.

Such studies have been helpful in identifying the natural course of diseases, in recognizing the emergence of epidemics, and in recognizing uncommon risk factors. It cannot be used to test for a valid statistical association, but can provide the basis for further case control studies, where the patients in the case series can be used in the control group.

A case series can be consecutive or nonconsecutive.
- Consecutive case series is a clinical study that includes all eligible patients identified by the researchers during the study registration period.
- Non-consecutive case series includes only some of the eligible patients.

Adapted from National Cancer Institute at the National Institutes of Health. NCI Dictionary of Cancer terms. Accessed on October 7, 2012.