In patients with phenylketonuria (PKU), extra tyrosine supplementation is advocated in addition to tyrosine-enriched amino acid mixtures. PKU patients have low fasting plasma tyrosine concentrations, but little is known about tyrosine fluctuations during the day. Plasma tyrosine concentrations were studied in 12 PKU patients in response to a test without breakfast and to three tests with different tyrosine contents in breakfast and lunch: 0%/30%, 25%/30%, 50%/10%, and 75%/10% tests, reflecting the protein consumption at breakfast and lunch, respectively. Prolonged fasting resulted in a small decrease in the already low overnight fasting plasma tyrosine concentrations. Breakfast and lunch with 25% and 30% of the daily tyrosine intake resulted in both lower than normal and higher than normal tyrosine concentrations. The 50%/10% and 75%/10% tests resulted in excessively high plasma tyrosine concentrations in most patients. Therefore, both lower than normal and higher than normal postprandial plasma tyrosine concentrations were found in treated PKU patients, even if the daily tyrosine intake was distributed evenly. When there was a large fractional tyrosine intake from one meal, very high plasma tyrosine concentrations were found. Therefore, strict control of plasma tyrosine is necessary if tyrosine supplementation is considered in addition to the tyrosine-enriched amino acid mixtures.