This study will test whether minoxidil taken by mouth (oral minoxidil) can improve hair loss caused by cancer treatment in children and young adults. In addition, the researchers will test the safety of oral minoxidil, and see if the study drug causes few or mild side effects in participants. Other purposes of this study include looking at whether participants are able to follow their study drug dosing schedules, and how oral minoxidil affects participants' quality of life.

Inclusion Criteria:

• Patients must have been diagnosed with cancer before the age of 17.
• They must have completed either systemic or radiation therapy (cytotoxic chemotherapy and external beam radiation therapy) for their cancer at least 1 year prior to study entry.
• They must be between 6-18 years old at the time of enrollment.
• They must have a clinical diagnosis of persistent or late alopecia for >6 months and that is definitely, probably, or possibly related to prior chemotherapy and/or radiation.

Exclusion Criteria:

• Had a history of alopecia prior to systemic or radiation therapy for cancer, or has alopecia unrelated to cancer treatment.
• Have scalp disorders that preclude the evaluation of alopecia, such as psoriasis
• Has a known hypersensitivity to minoxidil
• Concurrent use of other therapies for alopecia
• Concurrent active anticancer therapies (cytotoxic, targeted, endocrine, immunologic)
• History of chronic sclerotic cutaneous GvHD affecting the scalp
• Active chronic cutaneous GvHD
• History of orthostatic or symptomatic hypotension, syncope related to hypotension; systolic less than or equal than 100 (for adults) and 90 (for children) at screening.
• Pregnancy.
• Cardiovascular disease that in the opinion of the cardiologist makes the patient unsuitable for therapy
• Blood pressure less than the 5th percentile or less than 90/50 mmHg for children 10 years or older

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.