Our study was designed to measure the transcutaneous PO2 of the scalp to determine if there was a relative microvascular insufficiency and associated tissue hypoxia in areas of hair loss in male pattern baldness. A controlled prospective study was performed at Butterworth Hospital, Grand Rapids, Michigan. Eighteen nonsmoking male volunteers aged 18 years and older were studied. Nine men had male pattern baldness (Juri degree II or III), and nine were controls (no male pattern baldness). Scalp temperature and transcutaneous PO2 were obtained at frontal and temporal sites in each subject. Peripheral circulation was assessed from postocclusive transcutaneous PO2 recovery time by means of maximum initial slope measurements. Statistical significance was assessed at p < 0.05. There was no significant difference in scalp temperature between male pattern baldness subjects and controls. Temporal scalp blood flow was significantly higher than frontal scalp blood flow in male pattern baldness subjects; however, there was no significant difference in controls. Transcutaneous PO2 was significantly lower in bald frontal scalp (32.2 +/- 2.0 mmHg) than in hair-bearing temporal scalp (51.8 +/- 4.4 mmHg) in men with male pattern baldness. In controls, there was no significant difference in transcutaneous PO2 of frontal scalp (53.9 +/- 3.5 mmHg) and temporal scalp (61.4 +/- 2.7 mmHg). Transcutaneous PO2 also was significantly lower in the frontal scalp of male pattern baldness subjects (32.2 +/- 2.0 mmHg) than in either frontal or temporal scalp of controls (53.9 +/- 3.5 mmHg and 61.4 +/- 2.7 mmHg, respectively). There is a relative microvascular insufficiency to regions of the scalp that lose hair in male pattern baldness. We have identified a previously unreported tissue hypoxia in bald scalp compared with hair-bearing scalp.