diff --git a/src/flags.ts b/src/flags.ts
index c54ca01..c49169b 100644
--- a/src/flags.ts
+++ b/src/flags.ts
@@ -10,6 +10,10 @@ export type IFlagBase<T, I> = {
   description?: string
   hidden?: boolean
   required?: boolean
+  /**
+   * also accept an environment variable as input
+   */
+  env?: string
   parse(input: I): T
 }
 
diff --git a/src/parse.ts b/src/parse.ts
index 62bf074..ed1cdd3 100644
--- a/src/parse.ts
+++ b/src/parse.ts
@@ -172,12 +172,17 @@ export class Parser<T extends ParserInput, TFlags extends OutputFlags<T['flags']
     }
     for (const k of Object.keys(this.input.flags)) {
       const flag = this.input.flags[k]
-      if (flags[k]) continue
-      if (flag.type !== 'option' || !flag.default) continue
-      if (typeof flag.default === 'function') {
-        flags[k] = flag.default({options: flag, flags})
-      } else {
-        flags[k] = flag.default
+      if (flags[k] || flag.type !== 'option') continue
+      if (flag.env) {
+        let input = process.env[flag.env]
+        if (input) flags[k] = flag.parse(input)
+      }
+      if (!flags[k] && flag.default) {
+        if (typeof flag.default === 'function') {
+          flags[k] = flag.default({options: flag, flags})
+        } else {
+          flags[k] = flag.default
+        }
       }
     }
     return flags
diff --git a/test/parse.test.ts b/test/parse.test.ts
index b806b9f..049a349 100644
--- a/test/parse.test.ts
+++ b/test/parse.test.ts
@@ -371,4 +371,15 @@ See more help with --help`)
       }).to.throw('Expected invalidopt to be one of: myopt, myotheropt')
     })
   })
+
+  describe('env', () => {
+    it('accepts as environment variable', () => {
+      process.env.TEST_FOO = '101'
+      const out = parse([], {
+        flags: {foo: flags.string({env: 'TEST_FOO'})},
+      })
+      expect(out.flags.foo).to.equal('101')
+      delete process.env.TEST_FOO
+    })
+  })
 })