There could be circumstances in which it's legal. Nothing about rent stabilization makes rent increases mandatory and there are a few situations where a landlord can be selective. Are some of the leases being renewed or converted to month-to-month rental agreements? If so, those tenants might get rent increases but people whose leases have not yet expired cannot. Could it be that the tenants who are receiving no increase are already paying what the landlord considers to be a reasonable market rate, while people whose rent is lower are receiving increases to bring their rent more into line with what the others pay? A building may be stabilized but not have all the apartments in it covered. If the building turned into a co-op or condo, for example, or if somebody's rent rose above the $2000 limit and that tenant applied for the apartment to be deregulated, not all units may be stabilized. Is it possible that what you have is a rent controlled building as opposed to a rent stabilized one? If so, the only way to keep the rent control is to occupy the apartment continually since early 1971. People who move in after that don't get rent control. Another thing that comes to mind is the case when not all the apartments are the same... maybe some are 1-room and others are studios. In cases like that the rent increases aren't necessarily uniform. Same goes for when someone moves out and another tenant moves in. The landlord must disclose the old rent, but he's not required to disclose the rent being paid in all the different units. So the new tenants might get an effective increase (or a discount if it's a renter's market!) while the old tenants continue with whatever their lease dictates. Now, if all the leases are expiring, all the apartments are identical and under the same rent stabilization rule, all the rents are approximately the same, and some people are having their rents increased while other people are having their rents kept low, that's not OK and it's usually a sign the landlord is trying to get rid of someone. It's also not OK if rents are being raised based on ethnicity, etc. in violation of the fair housing laws. Disclaimer: I'm not a lawyer but I've been a landlord for years.

What should be illegal is rent control.